The Keys are incredible. Drove from Maryland to Key West on my honeymoon and had such a blast in Key West that we've been trying to get back ever since.
If you're into the big scene and want to party it up, I'd recommend staying directly in Key West (My wife and I stayed at
Lighthouse Court, directly behind Ernest Hemingway's house...when you look at the link, the picture that comes up, our room door is the one on the right hand side near the pool...it was very quiet there, even though it was pretty full). If you want something a bit more quiet, check out
Big Pine/Lower Keys or Marathon. One of the daily things to do in Key West is the Sunset Celebration at Mallory Square. Definitely check out the sunset cruises, they are a lot of fun.
You can do a 7 day cruise in the Caribbean in your price range. The one place I've always wanted to go to is Cabo San Lucas.