I found this fun art project here, and since rainy days have not been in short supply we tried it out. So cool!
You just drip food coloring onto card stock, then set it out in the rain and watch the rain splatter the drops.
We brought it inside, let it dry, and now I just have to decide what to do with it! I think it would be cool to mix some food coloring shades and do the drips in a more deliberate shape, or you can embellish your splatters like in the tutorial I linked to above. I'm thinking of bright colored mats and simple frames, then hanging them in the playroom. Other ideas?