Thanks for the ideas. We may definitely go without favors at all, but I do like the idea of giving them something to do for the hours the waterpark is closed and they are supposed to be quiet in the hotel room. Although, I'm a bit scared to give them fabric paint and nail polish in a hotel room - I'm having nightmare visions of charges for carpet stains.
I was planning on making a collage of pictures for the front of the thank-you cards, but I could frame it instead - maybe magnetic locker frames.