We let the girls out of the foster room to let them roam all around downstairs (setting up a gate - aka, a large box - at the bottom of the stairs, so that Yoko is separated from them), and just left blankets and pillows and boxes and toys all over the place for them to play with. And are they ever having a great time down there!! I keep hearing boxes being toppled over and jingling toys being tossed around.
I also have a big foam mattress I use to put on the pull-out couch when guests come over (the actual mattress on it is pretty hard), and I put that out and leaned it against the couch so that they would have a soft climbing wall. Oh my gosh, they just love love love it!