I'm not sure about the other ships, but on the Wonder and Magic they will often have late night snacks available in the adult pub (Crown and Fin or O'Gills) for an hour or so around 10:30-11pm, the quick service by the pool is usually open until around 10pm, the pizza place is open for a bit longer than that, and the ice cream station is usually available until 10-11pm. The snack stand by the movie theatre will have popcorn and chocolate bars (for an extra charge) and is open about 1/2 hour before each movie, even the ones late an night, so that's an option too. No late night buffets anymore.
Aby