Renting down south would be better. Possible weather issues even in NJ may influence your departure date otherwise.
Bahia Honda...even if you go for just the day is a very nice park and they have kayak rentals, concession, various "beaches" to explore.
Long key is a strip of a park really close to the road...if you can block out that part it is nice also. Curry Hammock is further from the road and small..kayak rentals there as well.
Key west deserves a day to explore and experience.
Sounds like you won't have a tow car so that would prohibit you from visiting pidgeon key (unless you stay at the campground that is close to it or bike to it). We biked there from Jolly Roger but it was not just down the street (20+ round trip). Oops, you probably won't have bikes either....
You can go to any of the state parks as a day visitor...although day visitor to Long Key is not very interesting. Curry a bit better. Bahia will give you more options.
There are more primitive campgrounds in the Everglades as well that are worth a look at if you are heading there from say tampa...not so much if it is Miami...unless you have time.
Eating: keys fisheries, Square Grouper (get my vote because it has gluten free menu and it is just tasty no matter what you get...lunch is cheaper and time it right), Big pine restaurant has good ribs and other burgers and such.
Two is probably a nice enough amount of time if you don't have to drive from NJ...otherwise it is not enough.
By the way, used to go to Bertrand's Island as a kid from White Meadow Lake in NJ...happy times!
