A Tree House!
I think every kid loves a treehouse. I didn't grow up with one but always romanticized about how fun it would be to have a fully customized dwelling in the back yard that was just mine. I would have requested a rope swing, rope bridge, fire pole escape, sleeping loft, hidden rooms and compartments, rope & bucket, trap door, electricity for the small refrigerator and record player, a crow's nest, rope lights and of course a sign that read, "No boys allowed! Ever! (except for Brad)".
Recently, I have seen some posts on Facebook of the most incredible living spaces that are in fact tree houses for adults. Bucket List?