I guess its all according to what you're going for. Obviously that little town is gonna be rock'n this time of year ... schools out, etc.
If you're looking to party, Sloppy Joes and the Lazy Gecko are 2 bars that gather large crowds, and they're right next to each other on Duval Street.
Some of the more fun parties go on at a few of the motels ... the Best Western Key Ambassador usually has some good partying around the pool; if you're there more than an hour you're sure to see some tits as the ladies tend to ditch their tops when the parties get hot ... 'bout 10PM or so.
If you're looking to "hook up" with some pecan tan bull, you might do better going back to Miami Beach this time of year.
If you're a diver, there's a cruise out to the coral reefs and to the Atlantic 'shelf' that are fun ... Mac