Anything!
Although disneyland.com doesn't have a page for them, if you search for the phrase you can read about them on their little "sidebars".
Here's one very informative sidebar:
All shops in the theme parks and Downtown Disney® District also accept Disney Dollars.
For the full Disney experience, use Disney Dollars while dining or shopping for Resort purchases! Disney Dollars are available at any
Disney Store in your mall or at the
Disneyland® Resort.
Can also use them at the carts that don't accept cards.
And if you bought too many and don't want to save them for a future trip, take them to a gift shop and have them swapped back into cash!
Their ONLY problem is that if you lose them, they are GONE. You can't get them replaced at all.
And the not-good/not-bad thing about them is that when you use them, you are given normal change, not disney dollars or disney coins, LOL. It made my budgeting with them weird, b/c suddenly it was just cash, not Disney Dollars, and it was harder for me to keep exact track of what I was spending.
I budget at the parks by buying Disney Giftcards for most everything, and 10 or 20 dollars in Disney Dollars for the carts that can't take cards or giftcards.