But the one thing they always come up with are books. Hardcover books that they have been granted permission to do whatever they please with. This is an absolute thrill to them, and I can see why. It's the joy of being given carte blanche to something that is usually to be treated with care and respect (careful, that's a library book!). And what they come up with is great...
Sometimes they write complete stories.
(some of them are suspense thrillers)
Sometimes they use the books like you would use a sketchbook, and doodle on each page.
The girls will sit for long periods of time with these books and write, draw and scribble in them just because they can. I save all of them, and the great thing is that all this work is bound neatly together already—no loose papers to gather up and go through.
Buying hardcover books would be a good purchase for the kids at a yard sale (instead of that skanky Snoopy stuffed animal they inevitably want). Tell them it's theirs to do whatever they please with, take it home, and watch their newly-granted licence inspire them.