The beach is the must thing. One of the best beaches in the whole world IMHO. Plus you'll have it all to yourself because the locals think it's too cold.
But if you get tired of the beach, Busch Gardens in Tampa is fun. Theme Park with great roller coasters and a great natural preserve (you know, like a Zoo, only huge enough that you don't feel sorry for the animals).
When I'm in Clearwater Beach, I always go to Frenchy's for the She-Crab Soup. I don't know why I love it so much.
If you're in for some cheesy entertainment and all the beer you can drink, take the nighttime Captain Memo's pirate cruise.