It all depends on what you like to snack on, how old your kids are and where you are staying.
I like to grab some juice boxes, beer, wine, milk and OJ to drink. I also buy apples and bananas that do not need refrigeration, individual snack packs of crackers or cookies, microwave popcorn, and Dole fruit cups. The juice boxes, snack packs and fruit cups come into the parks with us. You can also buy bigger packages of snacks and put them into ziplock bags you brought with you.
If you're at a resort with a food court you can also buy bagels and cream cheese for breakfast. For instance, toasters and microwaves are available at the All Star resorts that you can use. You can buy a styrofoam cooler at your grocery stop to keep things cold, but we bring a collapsable cooler.
Given that you only have 15 minutes, I would make a list before you get there!