We tried a few different mediums with our water colors. Some worked and others, well the mess they made was fun.
We used string, a straw, and cotton balls. He enjoyed the challenge of dipping the straw into the paint water, sealing the top, and then letting go over the paper to make a drop. The cotton balls turned into mush, but A liked them a lot (as you can see by his discolored fingers). He couldn't do a whole lot with the string, but I would make a long stroke and then he would use the straw to put a drop of different color at the start of the stroke and then watch it bleed down the line. (Does that make sense, it's a hard process to explain.)