I usually have about $100 in cash for DS and myself, with a lot of it in smaller bills for tips or for spending money for DS. I give him a set amount each day of the trip, and he can choose to spend it on whatever he wants (treats or toys or whatever) or save it each day to spend it all at the end of the week. I hang onto it so he doesn't lose it, but every morning we count it out into his envelope and talk about how much he has left. When he was a preschooler, I just gave him $5/day, but this year he's working on doing extra chores each week so he can get up to $10/day.