Tree house.

Deep in the forest there's a huge tree house that someone build and abandoned, for some reason. It's seriously like the Swiss Family Robinson's house. There's planks of wood nailed to a tree that you can use to get into the main portion, but it's quite a ways up so be careful. If you're more agile, you can simply use the large branches of the tree like stairs and make it up that way instead.

The main portion has closed walls and a wrap around balcony with railing, and a pretty well built and water proof ceiling. Other parts branch off with wooden bridges connecting them, or sometimes just a zip line. The other "rooms" are smaller, but also with thick walls and well built ceilings. There's a rope hanging down off of one deck like portion that you can use to swing into a lake that the house is leaning over. Or, you could simply dive off the deck.

At the top of the main building is a smaller building, almost like a watch tower. It's a little difficult to get to though. There's no real clear way to reach.