Mix a little white glue liquid watercolor. Have the kids use droppers or pipettes to place drops of rain across the top of their paper, then hold the paper up to make the rain flow down the page. Add cotton ball clouds while the glue is still wet.
Another fun activity for a rainy day is to open an umbrella, have a few kids get under, have a few others use spray bottles with water to make it rain. Don’t let them get too carried away though, because soon they’ll all be drenched!