Use your snack credits to get things like milk and water (if you need/want these items) And also check at your resort's gift shop to see if they have big bottles of these items. We stayed at Coronado Springs and I found that, while you can get individual serving sizes of milk, water and juices for snack credits at the convenience store in the resort, you could get much larger bottles of milk, water and juice for the same snack credits at the gift shop.
I also agree that you should use your snack credits for as much as possible during your trip. On our last night, we found ourselves with 11 extra snack credits so we ended up buying boxes of cookies and candies just to use up the credits. By the way, you can get nice 7 oz boxes of Minnie's Bake House cookies for the snack credits and they make nice souvenirs/gifts. Just keep in mind that every store seems to have different rules regarding snack credits. In Downtown Disney, I was told in World of Disney that I could not use a snack credit to buy these cookies. Then, in another store, I was told that the total price, including tax, had to be under $4...and since the 7 oz boxes were $3.95, they didn't qualify. But then in CSR's gift shop Panchito, I was told that as long as just the price itself was under $4, it qualified for a snack credit. So be sure to ask around if it doesn't work the first time.
HLDisney (just back 2 days and wishing I was back there again....)