Last December, when I was in Miami, it was 90 degrees! I was walking around in a tank top and jeans and regretted the jeans. Definitely dress for heat and be prepared for rain. Sunnies, suntan lotion, swimsuit and sundresses
What AJ said, enjoy the beaches during the day. If you will be staying in one of the hotels on Ocean Dr., it is blast just lounging in the downstairs restaurants sipping Mojitos, listening to live music and just people watching. For activities, you can rent bikes and ride around, go para-sailing, sky diving - whatever tickles your fancy. If you prefer something slower paced you can take in the architecture and history of the art deco district with or without a guided tour. Also, if you are going to be spending a weekend there, I highly suggest Sunday brunch at Nikki Beach (close to the Marriot at the very beginning of Ocean Dr.) It could be a little pricey (the brunch is not too expensive but the drinks add up) but it is right on the beach and with live music and beautiful settings, it is just what one needs on a beach getaway Another excellent choice with excellent brunch food is Front Porch. There is always a wait but not for more than 5-10 minutes and the food is totally worth it! Also, if you can, do try the Mojitos at the restaurant in Hotel Cardozo. You might get a kick out of the HUGE servings
South Beach is a whole different story at night. As far as the clubs, I'm always underwhelmed. People are allowed to enter based on how trashy the women are dressed. Did it once and will never again. We try to dine atleast one night at Taverna Opa (right across from Nikki Beach), very lively and entertaining with great food and wine. I've been hearing a lot about a restaurant called Sugarcane (Tapas place) lately. Might be worth a shot. Though, I think you might have to drive out of South Beach to get there.
Let me know if you need any more info and I'll rope in my cousins and friends to get answers!