One of S's favorite watercolor projects is a bit of a game. I sit with a white crayon across from her and she describes a crazy picture. I listen and create what she's describing with the white crayon on the white watercolor paper. (This is hard, but if you angle the paper a bit, you can see the white crayon more easily as you draw.) Then using a few colors of liquid watercolors (pink, yellow, and green), S paints over the picture revealing each of the aspects of the scene she described.
If you don't have liquid watercolors, regular watercolors (the kind in dried oval cakes you add water to) will work just fine.
I try to spread out the elements of the picture she's describing, and sometimes I add a few words, her name or something else as a surprise.
Here are two completely revealed examples. The top is S riding a tame polar bear who is stopping to take a nibble from a flowering shrub at night (of course). And the bottom is a scuba diving mouse. I enjoy this activity as much as she does because the things she imagines are just too hilarious. I also just love hearing her squeals of delight as she reveals her hidden picture.