TS restaurant are about the only place you can't use a snack credit.
You can use them at CS place, bakeries, carts, stands, resort grab 'n gos, candy stores, etc.
I think there are some restriction at some places on the Boardwalk, but most places can sell you say a bottle of Coke and that's a snack credit.
Just be sure you ask before buying or promising. For example, the fill your own candy wands (ie. pixie sticks) have two sizes, the small was a snack last year, but not the large. So don't promise the large before you check; it will keep the whining to a minimum.