Kids seem to love tents. If the temperatures aren't too hot/cold/windy they can make cheap and easy shelter for them.

Don't overlook that discount store dome tents are much better than the near useless pup tents sold twenty years ago. Ironically the inexpensive dome tents are not much more expensive than the pup tents used to be. If you can catch them when they are changing out stock and on sale you can get a lot of tent for short money. I wouldn't want to try to tackle K2 in a discount store tent but for a week in the backyard they are perfectly adequate.

Mattresses last longer and stay fresher if they are flipped and aired regularly. Back into the 50s it was standard practice to flip the mattress every time the linens were changed and to take them outside to air in the sun a couple of times a year. No reason why the same things could be done today. With a crowded house there are plenty of hands to make the job easier.