Have children draw pictures with water soluble markers on white paper. Fill a spray bottle with water and have them make it "rain" on their picture. What happens? Dry flat.
Rainy Day Fun
Have children brainstorm things to do on a rainy day. Let them all draw a picture of their favorite thing and put them together to make a class book.
Rainy Day Reading
Get a large golf umbrella and open it up in a corner of your room. Place several "special" books under the umbrella and allow one or two children at a time to sit there and read.