Ice Cube Paints!
A quick Pinterest search will give you lots of different ways to make these. There are versions floating around that include mixing paints with water and mixing food coloring with water. We always have food coloring in their cabinet so we went with that option and it has been perfect! Apparently washable paint is the best option to reduce staining but we've been using food coloring and have had no issues (although we usually do this activity in a bathing suit or birthday suit...).
Just mix 2 or 3 drops of food coloring with water in an ice cube tray with some popsicle sticks (no need to even stand them up straight) and freeze. When they are frozen, they are ready for painting!
We've been painting plain paper, rocks, shells, tree stumps, whatever we can find. This makes for a great outdoor activity on a warm day. A little messy but very fun!