Good website to get started
https://cincinnatiusa.com can also request a free visitors guide be mailed to you from that site.
Kings Island, Beach Waterpark, and Miami Valley Gaming are all nearby the tennis complex.
Cincinnati has tons of historical stuff as we’re known to be America’s first city (first city founded after the Revolutionary War by American citizens instead of European colonists). OTR is a great place to check multiple boxes if you only have a short time to sight see. Also the neighborhoods architecture is awesome especially Music Hall and Memorial Hall. Washington park is cool to checkout. Add in restaurants and bars and you’re set and these things are within a few block radius as is the underground tour. A few blocks north of there
Findlay Market is cool to checkout along with a couple breweries.
A little south into downtown (walkable or can take the streetcar)
Fountain Square is always popular. And of course
The Banks district between the stadiums is a popular place to eat and drink and leads right into the riverfront park which is highly recommended.
Other options would include the Mt Adams neighborhood just east of downtown that has popular restaurants and bars along with impressive views since it’s way up on a hill. This is also where you’ll find Eden Park with the
Cincinnati Art Museum and Krohn Conservatory.
Cross the river and checkout
Mainstrasse Village in Covington or
Newport on the Levee.
Hope this helps, have a great trip!