I like this one: If you fill your air mattress with helium you can keep it on the ceiling.

William Calderwood’s 1989 brainstorm automatically increases the floor space in a small apartment. When you get up in the morning the bed floats to the ceiling, and you can spend the day roller-skating beneath it. Then at bedtime you pull it down again by the tether. Best of all, you never have to make the bed, because no one will ever see it!