We love Bahia Honda and camp there every year. It's hard to get a reservation there so good luck. The Atlantic and the Gulf meet and it's a beautiful park. There are 2 main loops for camping. Buttonwood has some bigger sites and a little more privacy. If you lucky enough to get a waterfront site you can snorkel off your site or launch your kayak. We float on rafts after a day of fun and cool off. You see a good sunset here too. 

The sandspur loop is for campers under 23' and they are tighter with no shade or privacy. Yet they are right on the Atlantic ocean and the view is awesome. Loggerhead beach is beautiful. 

You can rent a kayak if you don't have one and paddle around the island. It's all good. The old Flagler bridge is a great photo op and you can walk up to it and catch a beautiful sunset or stargaze. 

There is a primitive campground  loop too on the gulf side near the Cabins. You can't get under the bridge to drive back with a high vehicle so make sure you look at the standards if you are booking a cabin or a primitive site. My truck with the kayak racks on top won't go under it. 

Lot's of good eats south on Big Pine Key and north on Marathon. There are 2 grocery stores on Marathon Key and a Winn Dixie on Big Pine Key. Lot's of small deer on Big Pine Key do not speed they will get you. As a matter of fact the keys are loaded with police looking for speeders so take it slow.