In the summer when the temperatures reach sweltering and the city is crowded with tourists, the Boston Harbor Islands are where I go to escape. There are 12 islands to visit in total, and although a mere 15 to 20 minute ferry ride from the wharf, you'll feel worlds away.
You can purchase ferry tickets at the kiosk at North Long Wharf. My advice is to check and double-check when the last ferry returns to Boston to avoid rushing to catch it. I started mid-morning so I had plenty of time to enjoy two islands in one day. If you're a movie buff, you'll think of Scorsese's "Shutter Island", which doesn't actually exist (though some filming was done on Peddocks Island).
On three of the islands, you can pre-book to camp, and while I haven't tried it, the thought of playing castaway for the night sounds like my kind of adventure.