I have read that some pack kids clothes in them. Each bag has an outfit...shirt, socks, underwear, shorts...that way they simply pull out one of the bags each day and give to the kids. I imagine the gallon sacks would be large enough.
I use them for anything that would leak. Although I have a separate bag for bathroom stuff..I still put shampoo in a plastic bag and then put in there. It has saved me many a mess. Also, on the way back, I use it for Mickey shampoo. It leaks terrible.
At the park I use them for snacks, wet wipes (when there isn't a trash can), diapers (when there isn't a trash can), and like someone said to put things that you don't want to get wet in them. Works great on water rides.