Summer Days in Tampa

Its soooooo hot outside in the summer. I know on my days off I like to go to the pool or the beach but sometimes that can get boring. What are some other fun things to do outside that can cool you off that doesnt cost alot of money???? I know adventure island but how much does that cost? Does anyone know how to get discounts? Also I have heard alot about float trips down some of these freshwater rivers that are crystal clear...does anyone know anything about that??