Hitchhikers are rare in Florida, but it's still possible to get rides. It's best to stick to the Interstates; you won't have much luck on US highways and State Roads because so much of Florida is suburban, and most people on these roads aren't going far.
If you want something fun to do outdoors, going to a spring can be fun. They're all over the state, very cheap, and usually not very crowded. They can sometimes be off the beaten path, though.