A cardboard box.
I work in a medical practice and we just had a new examination bed delivered. It came in a cardboard box with 1cm thick walls. I saw it just standing up against the wall, ready for recycling. It's very long, big enough to stand inside.
Suddenly I was 6 years old again. I could see the tall box as a coffin, a phone booth, a pirate ship, an egyptian sarcophagus, a hide-out ...
Made me wish I was 6 again, for just a little while.
Made me happy.