Crandon Beach Park is a sick spot located on Key Biscayne just south of downtown. The place can crank in the fall and spring and sporadically during the winter. It is a definite scene. Lot's of kiters and watch out when the tide get's low, It turns into a zoo of people boosting over the resulting sandbar. Way on the outside when the wind cranks, you get good waves to ride. Make sure and check with the locals at this place as they are heavily regulated at Crandon.The local shop is Waterplay
There's endless other things to do in the area and the downwinder possiblities here are sweet.
Check out pictures of Crandon here