All of the previous suggestions are good ones. Since you will be on Oahu, I recommend the following:
Places to get to:
1. Lanikai. My favorite spot in the world. Picture post card perfect. Windward side, in Kailua. Gentle surf. Rent a kayak and paddle out to the Mokes (Mokulua Islands), just a quarter mile off shore. The island on the left (Two Hump) has a nice beach. It used to be very uncrowded. I have seen it become more crowded lately. Still awesome. There is a crashed WWII P-40 fighter just east of One Hump (the island to the right). It is in about 15 feet of water, an easy snorkel, but not easy to find. It was shot down during the Japanese attack on Pearl Harbor. While at Lanikai hike the pillbox. Short, easy hike and a stunning view!
2. North Shore. Still a little winter surf hanging around. Watch the undertow at Waimea. Get there early or you won't find parking. Be sure to jump off the rock. You might want to hike Waimea Falls nearby while you are in the area. Check out the whole North Shore while you are out that way - Banzai Pipeline, Sunset Beach, Waimea. Check out Haleiwa, cool little surf town. Be sure to stop at Matsumoto's Grocery for the most famous shave ice in the world. Mackey's Shrimp Truck in a gravel lot in town has an awesome garlic butter shrimp plate for about $12. At the west end of the North Shore is a nude beach at Mokuleia, if you like. It is near the tiny Dillingham airport, where you can sky dive if you want. You walk through a horse pasture to get there. Take some apples or carrots and feed the horses. When you get to the beach, the beach of the all-over-tan is on the right. On the left is a more PG beach. You will likely encounter people on horseback riding along the beach. I forgot to mention a million things to see on the North Shore, but snorkel Shark's Cove, near Sunset Beach if you get the chance. Better than Hanauma, in my opinion, depending on surf conditions. We are still getting a bit of winter surf, so even though it is within a protective cove the surf outside the cove could make it washing machine conditions - not good. Odds are it is OK though. Farther to the east on the North Shore is Kahuku. Lots of shrimp farms and home to Giovanni's Shrimp Truck. You can get a delicious shrimp plate and sign the truck. It has thousands of signatures on it. Bulldog is on it several times (LOL!) Try to drive the North Shore during the week. On the weekend the traffic can get pupule ("crazy" in Hawaiian), as it is just a two-lane country road, one lane each direction. Laniakea Beach is cool also if you want to see turtles. It's nickname is Turtle Beach. Turtle haven.
3. Of course, Waikiki is a must. Lots to see and do. Shopping, beaches, 5-star restaurants. My fave eats in Waikiki - Lulu's on the zoo end of Kalakaua. The Kalua Pig Nacho Plate is great and can feed two for about $15. Cheesecake Factory is the most popular restaurant in the state. My son is a server there. I think they seat close to 1,000 people. The typical wait is about an hour and a half. Duke's, especially for Sunday Brunch ($15), is terrific. Right on the beach. Visit the Shorebird just down the street, also on the beach. They have a ceiling fan system that is amazing. It is from the 1920s or so. There are like 30 ceiling fans all run on a single motor and big ass rubber band. Pretty amazing. Looks like something out of Casablanca. Across the street is Jimmy Buffett's restaurant. He gets on stage and plays for the dinner crowd when he is in town, once a year or so. Waikiki beaches are crowded but not so crowded you can't enjoy yourself. I saw much larger crowds at Myrtle and Va Beach. We locals like to hang out at Ala Moana Beach, just outside Waikiki, adjacent to the Ala Wai Yacht Harbor. Much less crowded, especially during the week. If you go on the weekend, get there early for parking, as the local kine are setting up tents and canopies. Very gentle surf, inside a protected reef. Set yourself up at the east end near Magic Island. The west end is rocky. Fireworks at the Hilton Lagoon across the harbor on Friday nights at 7:45 sharp. Nice display, courtesy of the Hilton family. You can go to the lagoon and lay on the beach and be practically right underneath the fireworks. Check out the sunset cruise on the Booze Cruise. $35 gets you an hour and a half sail and all you can drink, and they will serve them to you, and serve them strong, as quick as you can drink them. I have lost some brain cells on the Booze Cruise. Whale season is about over. You may still see a whale or two but they are getting more scarce. I saw a couple from my front lanai last week playing just outside the harbor. From December to March they are plentiful. Now is the time they start their migration back to Alaska. You might still see one or two though. There are three or four booze cruises. One is much better than the others. I think it is the Na Hoku II. My wife and I haven't been on it for two years or so, but it is a really fun sail. Link....
Na Hoku II Lots of good eats in the area. The beach at the base of Diamond Head is pretty cool. A little hike, but you may see sunning seals, turtles, lots of locals, especially surfers, and nudity is not uncommon there (It's Hawaii. No one really cares). Breakfast at Snapper's (on Ala Moana Blvd) is $3.99 and pretty good. They also have daily dinner specials. For example, the Thursday special is a juicy burger, chips, and a 16-oz beer of your choice for $4.95. Across the street is the Harbor Pub. Good eats, cold beer, and crazy sailors. The Hawaii Prince hotel has a seafood buffet with king and/or snow crab legs, clams, oysters, you name it. Costs about $50, but very good. I have heard it is acclaimed to be the best seafood buffet in the world, but frankly, I think Captain George's and Captain John's, in the Tidewater, are better. My opinion. Want to eat like a local? I recommend U-Choice on Kaheka St. in the Don Quijote Shopping Center. Chnese/Hawaiian mix. Huge portions for about $7, a dollar less if you show up after 6PM. They close at 8. Visit Don Quijote grocery store at the other end of the shopping center while you are there. The best and cheapest place to buy souvenirs in my opinion. It is a very Asian supermarket. You like Spam Musubi, brah? Plenty at Don Q. Lots of good Japanese restaurants and Vietnamese. If you like Pho, I recommend Pho One on Kapiolani. Very good. Also, L&L is a popular local lunch plate joint. There are many of them on the island. The best I have eaten at is in Waimanalo. Keep that in mind when you are touring the windward side.
4. Downtown. See Iolani Palace. You will probably only have time for a drive by, but even that is nice. It is on King St., heading Diamond Head from downtown. The only royal palace in the United States. Chinatown is just west of downtown. Very busy, very Asian, very interesting. My grandkids freaked out watching Chinese guys unload trucks full of pig carcasses, head and all, gutted only.
5. East end. See Hanauma Bay. It is like snorkeling in an aquarium. Turtles, choke tropical fish of every kind, octopus, morays, you name it. It is a tourist attraction, so you just have to deal with that. But it is still nice. Get there in the morning. The afternoons get windy and the water gets murky. I was scuba diving just outside the bay once and ran out of air at 100', flipped on my reserve, thought I had more time to goof around, and ran out of air again. Oops! Stupid! I lived to tell about it. Stop by Blowhole. Take binoculars if you have them, to see any straggling whales. The little cove on the right side of Blowhole is the scene of one of the most famous love scenes in movie history, From Here To Eternity. Hike Makapuu if you have time (three hours round trip). Check out Sandy Beach, which we locals call Sandy's. It is the premier body surfing and boogie boarding beach in the world. Wicked shore break. Be very careful. The waves break right on the beach and they will mercilessly pick you up and slam you right on the beach. Lots of tourist casualties at Sandy's. A lot of fun just to watch the local pros. They make it look easy. It ain't! Nearby Makapuu Beach is more forgiving and much better for novice to intermediate surfers. This puts you back on the windward side near Waimanalo and the craggy Koolau mountain range, which you will probably recognize from Jurassic Park and other movies.
Of course, there is Pearl Harbor, the USS Arizona and the USS Missouri and a thousand other attractions to see. I would skip the leeward (west) end unless you have a lot of time.
Whew! I can go on and on. Here is what I would recommend on Day One. A driving tour, assuming you are quartered in Waikiki. Drive Kalakaua east, scooting along the shore and over Diamond Head. Nice views along the way. Continue on, working your way along the coast through Kahala and Hawaii Kai, one of the most exclusive zip codes in the world. You will pass Hanauma Bay, where you can snorkel and chill for a few hours. Continuing on you will pass Blowhole, Sandy's, and Makapuu. Continue on to Waimanalo, just a couple miles away. Turn into Bellows. It is an old air force base that is now used mostly for R&R for our troops. Super nice beach. Not crowded. On the weekdays the military uses it for training so it is off limits to stop on the beach. But you can still drive by and see it. On the weekends you can camp on the beach. Coming out of Bellows turn right and head to Kailua, another couple miles away. Make your way to Lanikai and spend the rest of your day. Kailua Beach is also very nice. It is where King Barrack Hussein hangs out when he visits. Drive across the mountain on the Pali Highway to get back home.
Day Two driving tour. Head west out of Waikiki on Ala Moana Blvd. Make your way to H-1, which amazingly is classified as an interstate highway. I have no idea where the bridge to California is, but whatever, pardon my digression. Take H-2 past Mililani and Schofield Barracks, home of the storied 25th Infantry Division, my first duty assignment after graduation from JMU. Continue on through the pineapple fields towards the North Shore. Head into Haleiwa and follow the two-lane road along the coast to the attractions I mentioned in paragraph 2., above. Depending on which ones you decide to stop at you will likely use most of your day. Keep driving past Kahuku, along the shore, and towards Kaneohe. You can take any of the three roads across the Koolau to get back to Waikiki - H-3, Likelike (pronounced Leekay-leekay), or the Pali.
From this two-day orientation you should get a good idea of what you like the most and can spend the rest of your time concentrating on those spots.
I hope this helps. I have just scratched the surface.
