Depends on what you're into. Tons of stuff down there.
Beaches for one. Hollywood beach is nice, long boardwalk. Miami, south beach is nice. If you go on the weekends you can watch the cruise ships pull out which is pretty **** cool because the channel is pretty narrow.
Dave and Busters if you're into gaming. They have a few sky dying places (inside and outside) as well as other types of family fun centers. Rope climbing, zip lining all the typical tourist ****.
Everglades does tours if you want to go out into the swamp. I probably wouldn't say summer is the best season but if you don't mind sweating go for it.
Hard rock casino.
There is a boat the leaves for the Bahamas that you can take, IIRC it leaves from West Palm Beach. I'm not sure if that is the same one that does gambling or not but I think there is one that does both.
IF you don't mind the drive and are science orientated NASA has a kick ass facility in Canaveral.
Otherwise check Trip advisor for stuff that might fit your wants.
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