Costco

Costco?!
Yes, Costco.
I suggest going when samples are amply available (usually after 11am to early afternoon). But that's not the reason I am posting about Costco. I love it because of all the free recyclables you can find!
Let your kids pick out some funny shaped boxes. Let them create a castle with them. Paint them. Turn them into stuffed animal furniture! Whatever you want. Last time we went we found these big cardboard panels. They are usually between layers of toilet paper and paper towels.
One of my kids turned it into a huge treasure map:

He did it in pencil so you can't really see all the detail. But it did take up a whole afternoon!
It would be fun to finger paint on something that big too!