Hello Mike,
For much of the Keys forget about the mainland concept of a sand beach. They do exist in the Keys, only rarely. In Islamorada there's Whale Harbor with a launching area and there is a fee to launch there. I think it's $10. It is a small place with no sand to speak of immediately adjacent to a parking lot. It does have a lot of flat shallow water however. Further south there is Anne's Beach still on Islamorada, again with no beach but lots of shallows. No fee to launch there currently. As I recall clear area to setup kites is pretty limited there. I would bring booties for the odd sea urchin or shell you might step on. The bottom is usually small areas of coarse sand and shell with most of the cover consisting of turtle grass.
I would suggest hitting Islamorada and running down the Keys to Key West for a trip to the flats with thekitehouse.com. You could stop by Bahia Honda State Park on the way north.
For diving I would suggest Atlantis Dive Shop in Key Largo. Been a long time since I have dove off Islamorada. There are dive charter operations throughout the Keys. I usually just go to Atlantis for diving when I'm down for that.
There are lots of night time options throughout the Keys. I think up a list and add it here soon.
Be sure to checkout the first two stickies at the following link for more ideas:
http://fksa.org/forumdisplay.php?f=89