Make the outside of the indoor "playhouse/room" to look like a real house. I would only of dreamt to have something like this as a kid. My kiddo and future kiddos deserve a kickass room they would be proud of.
My son has a fascination with the moon so this would be perfect picture this. a bedroom wall, painted in blue-black chalkboard paint, chalk drawing of the moon, and finished with a curtain of white holiday string lights.