Find a couch or low chair.
Lie on your back on the floor with your head right against the front of the couch. Tilt your head back so you're looking at the room from the ceiling-down. "Look — the room is upside down."
Giggling. "Now I'm on the ceiling!" Most kids spend 5 minutes naming what's on the (now-)floor and ceiling, then run off to find the next ceiling.
Hang knees over the couch arm so the head dangles (only briefly, and only for kids who can tell you when it's too much). Make up a story about "the upside-down world." Crawl through a chair-leg "tunnel."