When I was a kid, I liked the "spy" books that you read through and highlighted invisible ink messages with a special marker. It's not really a craft but it's definitely the easiest clean-up...none at all.
I would say pick an easy craft that has to do with your vacation destination. If you're going to the beach, buy small vases and have the kids collect shells or sea glass (not harmful, since it's smooth from the ocean) and make a cute arrangement they can put in their room. Plus, kids love to beach comb so they will be at it for hours.
If you're going out in the wilderness, the mountains, or camping...There's a really easy craft I did in girl scouts. Have the kids collect small but thick sticks, glue them together to make a picture frame, and put a picture from the trip in the frame when you're done!
If you're going to a new city or something of that sort, have the kids collect some things that will help them remember the trip-- tickets, stamps, anything small and available. There are tons of things you can do with this stuff...modpodge them to a small box they can keep photos in, glue them to a plain wooden picture frame, and much more.
Hope that helps!