Simple 1x cedar trim is great for dressing up a tree-house. After the door and window jambs are in place, cut trim pieces to fit along the top and sides of the openings. Miter or butt the boards together at the corners, and include a 3/4 reveal, if desired. Fasten the trim to the framing with galvanized box nails.
Install vertical trim boards at both ends of through walls, wrapping the trim around the end studs so it’s flush to the inside face of the wall. You’ll most likely have to rip down some of the trim to match the wall dimensions.