After a moment of thought, we decided on a pillow pile. We got every cushion, pillow, and comforter, and even an old mattress, and piled it high for them to jump on. The finished product was over 2 feet high and as big as a king-size mattress. They were ecstatic! They jumped and laughed to their heart's content. Several times they bounced and soared and convinced themselves that they could really, truly fly! :)
It required minor maintenance to make sure that it didn't get too spread out or too squooshed in the middle, but they were very cooperative about taking turns and following any safety limits (no flipping).Once they were worn out from all the jumping, it was easily transformed into the ideal snuggle-nest while we watched the Nutcracker.
We all need a soft, safe place to land and to snuggle after stretching our wings, don't we?