While at Salt Springs, there is a small marina where you can rent a boat for a reasonable price and explore the run down to the lake. Also, I'd recommend going over to Juniper Springs ( short drive from Salt Springs) and rent a canoe and do the run down the creek. It's 7 miles and a very scenic paddle.
Good luck getting in Ft. DeSoto.... they do have walk ups so maybe you;ll be lucky-- but you should have a plan "B" .
At Bahia Honda, go out to Looe Key to dive/snorkel. Sometime during your stay, head over to No Name Pub ( at No Name Key, off Big Pine Key) for pizza. Looe Key Dive Center also has a new,huge tiki hut with some pretty good happy hour specials. You should head to Key West for a day trip. Lots to do there..... if you like good live music, go to the Green Parrot. Great Happy Hour at the Boat House- 1/2 price small plates.