If you're driving in from Miami, maybe spend a day at Pennekamp State Park (Key Largo). Also Theatre of the Seas in Islamorada is fun, but can get a little costly with the "add-ons" (swim with dolphins, stingrays, etc).
If they want the Central FL theme park experience, and only have one day, I'd suggest Legoland in Winter Haven. If any of the boys or their younger siblings are in "Lego Club" their magazine has had BOGO coupons in the past.
Depending on what your Sea Base itinerary is, maybe consider a party-boat fishing day--thought that may not be a "hit" if you're already planning on doing a lot of fishing on your expedition.
Lots of fun touristy stuff in Key West if you want to drive down there then double back to Sea Base. Mel Fischer's treasure museum is always a hit, as are some of the "History of Key West" type of places.