If you are going to be on Miami Beach, you really should go check out the new TKS Miami store. It is probably the cleanest and best laid out store that sells kites that I have visited. Their Key Biscayne location is also quite nice but not as pretty as the one in Miami Beach. TKS also has the coolest logo of any kite shop out there so grab a hat or t-shirt.
If you do go to the Key Biscayne location, three doors down is a little restaurant called Patagonia, grab some food there!
If you have the chance, the drive to Islamorada, in the Keys, is worth it. Visit the Otherside Boardsports shop. They have a great crew there. There's a restaurant called MEAT nearby that has great BBQ and beer. If there's no wind they also have a cable park. Probably the nicest people running a shop that I have met in my travels.
With that being said, I do not have a kite shop where I live, so when I travel I try to go to each kite shop and I also try to make a point of buying something from each one. It's hard to run a retail shop. So if they are cool, I buy a t-shirt, otherwise maybe some lip balm, a sticker, or something small just to put money into their till.
Other places to see in Miami is Miami Kiteboarding at Crandon Park, on Key Biscayne. Not much in the way of a shop but they always have great staff from all over the world. Adventure Sports Miami is owned by Cabrinha's corporate parent and although they have a great selection of Cabrinha products, the store just felt like a big chain store to me.
Have fun in the sun.
One more thing....Jupiter, FL, has some great waves and downwinders if you're into that. it will also have some of the best riders around. Jupiter Kiteboarding is a shop worth popping your head into.