Gardening shed. Construct a cute garden shed in a weekend with a kit. Prefab wall panels go up quickly, and doors and windows slip into precut openings. Dress it up with hanging baskets suspended from beams and window boxes overflowing with flowers.
There was a super awesome pirate ship at a house near Snow College when I was there. I have wanted something similar ever since. Red Beards Revenge Pirate Ship Playhouse : Luxury Playhouses at PoshTots