We played with trolls in the winter and in the summer either played pioneers, explorers, or astronauts. A downed tree makes a dandy aircraft....lotta branches to use for levers.
At our first PA house, we had a huge backyard that sloped downhill steeply. At the bottom was an enormous fallen tree. The way the branches fell, there was a large open space in the middle that you could get to by climbing. We used that as a fort... sometimes roofed parts of it with big hunks of bark.