As a native floridian (actually living in london at the moment) here is what i suggest.

avoid orlando, unless you want to go to the theme parks. if you do want to go ask if someone at one your hotel or one of your mates can get you a florida resident discount. its much cheaper and not all the tourist know about the availability.

I highly recommend the diving/snorkeling in Key West. Key Largo is nice, but so are the reefs off of Key West proper. i would also recommend the Key West Diving Society dive shop for that sort of thing. I found them very professional and very nice in the past. Also in the Keys are some beautiful beaches (one, the island next to key west, was rated in the tip 10 of US beaches). so don't be in a hurry to just get down to key west. renting a convertible on the way down is a must.

I'm from tampa orignally and unless you fancy Bush Gardens, there is no need to go there. The water around tampa, st pete, and clearwater do not live up to the reputation. i find the water to be murky and poluted from the cruise and port traffic. But i would check out the south west coast. From Naples to Sarasota. sarasota is just south of tampa, but has some incredible beaches and has a very laid back florida feels to it. don't forget the ringling bros museum there. for ref, tampa and miami are about 5hrs drive apart, so keep that in mind.

its worth driving down alligator ally which is a section of I-75 that crosses the southern part of the everglades. there are literally hundreds of alligators sunning themselves on the side of the road. you won't forget it. you could also stop and go for an everglades airboat tour. be warned! you may get the feeling you are an extra in Deliverance. and you will know why bitt calls florida a "dump full of rednecks."

Don't forget the food, especially in the keys. IMO it is some of the best seafood in the world. i always make a point of eating well in miami and the keys. you are doing nothing but depriving yourself if you don't. Also, there is a rich Cuban heritage that goes back to the late 1800s in s. florida. i'm baised b/c i grew up on it, but the florida/cuban food is excellent (i appreciate its not to everyone's taste) and truely unique.

if you like to surf, or want to learn talk to the guys at the south beach quiksilver shop.

There is great fishing in florida too. There are restaurants that will cook up your catch. the taste of truely freash fish is completely different. you could even just book a boat ride around miami, you won't regret it. you can also go for picnics along small islands dotted around there. it would be an unique afternoon.

any more questions or specifics feel free to pm me.