boca chita camping
pack your mosquito spray, grab some marshmallows and Hersheys bars because we're going camping. Of course camping in Miami can be much more glamorous than anywhere else (my girlscouts troupe was far too prissy to really rough it so we stayed in the Disney campgrounds), and the nicest way to do it is, of course, by boat.
If you go to Boca Chita Island (are there addresses for National Parks?) I would go during the week to avoid the crowds and be sure that you are on the peninsula that you can see in the lighthouse picture so that you get a breeze from both sides. There are BBQs there and bathrooms but that's about it, so you do get the whole "great outdoors" experience. Growing up my brother called it Mosquito island so get serious about some bug spray and get one with a high DEET content.