One of my son's favorite things to do is paint with water. Although, he calls it "Brushing" Almost every day I hear "Mama, can I brush?"
I wish he was talking about his teeth or hair but he is not... he is talking about brushing water onto his chalkboard.
I have him "paint" right on his chalkboard easel. It is great because the chalkboard dries off and he can keep creating new pictures without me having to change out the paper. And lets not forget that water is MUCH cleaner than paint!!
All you need is a paint brush and a cup of water. I put very little water in the cup, in case it gets tipped over (that way I only have a little water to clean up)
As you can see, he gets pretty serious about it!
If you do not have a chalkboard you can also use construction paper. You can let the papers dry off and reuse them again!