The Eternal Tree House
This misnamed attraction is not a tree house, but a 20-foot-square room ("house") inside of a redwood stump. The room was made by a prehistoric fire, then hewed out like a Halloween pumpkin in the early 1900s.
The Eternal Tree House is free to view. Visitors step down into the sunken floor of the stump room, and can look up between a rustic log chandelier of four light bulbs suspended by chains up into the hollow center of what's left of the tree.