We vacation at the same campsite every summer, and our neighbors there always prop a little mailbox at the edge of their turf (and always get loads of letters from our kids). Last year, we finally brought our own. Homemade or store-bought, a mailbox makes a delightful diversion, breeds goodwill among fellow campers, and offers plenty of secret-admirer mystery (even if it's usually just doting parents writing those love letters).
Use the hammer and nail to make a hole in the can's side about 2 inches above the bottom rim.
Cut a rectangular flag from the extra lid, use the nail to make a hole in it near one end, and secure it to the can with a brad fastener (watch out for sharp edges).
Use bungee cords to secure the can to a tree or post, and leave a pad and pen inside for jotting notes.