Follow up ideas:
Make a rain stick.
Make a rainy painting by dropping blobs of shades of blue paint and tilting the paper to make the paint run in streaks.
Make a lightning picture. Use black paper. Dip a string in white paint. Drop it onto the paper. Carefully peel it off.
Draw a giant rainbow outside with sidewalk chalk. Watch the rain wash it away.
Make bowls of mud (chocolate pudding). Add sprinkles for rocks and a gummy worm.
Umbrella Exercise. Fold a colored paper plate (flimsy one) or paper circle into 8 sections. Put 1 raindrop on the first section, 2 on the next, until there are eight. Make a second umbrella. In each section write a movement: hop, clap, stamp a foot, touch your toes, kick, tip-toe, giant steps, march. Put the umbrellas on the floor. Toss two quarters, or small bean bags, or rolled up socks, one to each umbrella. Do the action such as hop 3 times.
Make a mobile of raindrops made from blue cellophane. Hang in against a window.
Read a rainy story like https://wp.me/p1OfUU-n0 Outstanding in the Rain, A Whole Story with Holes, by Frank Viva.
Go for a walk in the rain. Snap pictures. Make your own rainy day book.