Sometimes simple, classic art projects are the best. Case in point: blow painting. I don't know anyone who doesn't enjoy this.
All you need is watered down tempera paint (ink consistency), paper and straws.
Kids dribble a few drops of paint on their paper and then use the straw to blow the paint across the paper. You get lots of 'oooohs and aaaahs' from the kids when the colours mix together.