Depends on how much you want to spend and what you like to do. I am a few months shy of 28 and would suggest South Beach (Nikki Beach) to an out of town visitor 21-35, but be ready to drop some serious cash!
http://www.nightcure.com/Miami/clubs...ch%20Club.html
Diving is great in the keys also if you want to do something more relaxing.
http://www.southbeachdivers.com/