One Saturday morning, when my son was I woke up and built him a treehouse. That was one of the coolest things ever. I think I want to do it again, only with better design, and attach them with a zipline situation! That would be cooler. ;)
Recycle an old boat or canoe into a sand box! Drill a hole in the bottom of the boat to put the pole through. Make sure you bury the pole deep or concrete it in the ground. You could even make a sail out of tarp that can also double as the sandbox cover.