You have to do either SCUBA or snorkelling in the keys. Key Largo is the place to be for that - that's where James Bond always goes so it must be good . Seriously though, it is considered by the locals to be the dive capital of the World. If that's too much activity, then a glass bottomed boat trip is a (poor IMO) substitute. Maybe you fancy a days fishing. All of these are best done on Key Largo.
The African Queen boat is also on Key Largo IIRC. It wasn't open when I was there in 2000 though.

At Key West, the shipwreck treasure museum is well worth a visit. Lots of recovered valuables on display. Also consider a sunset catermaran trip that is a pleasant way of killing a couple of hours. If you're into literature then visit Hemingway's house. Some good bars in Key West. Check for any festivals - Key West can be a party town.

There's not any big beaches in the Keys. Small muddy bays amongst the rocks is basically it. So save your beach time for the mainland. Miami's South Beach is supposed to rock. Tampa Bay also has lovely sandy beaches with very warm water (Gulf of Mexico, protected from the Atlantic)

Orlando, there's obviously Disney World, Epcot, that big movie-set themed park (which studio...can't remember), Gatorland (worth a visit), and IIRC, a Busch Gardens.

The Kennedy Space Center isn't far away and worth the trip. Check the launch schedules online to see if anything is going up around that time.