Rain down an instrument
Make a rainstick for/with your child. Several sites have recommendations. A gentle reminder to supervise your child when they have a rainstick as it is made of small pieces and can be a choking hazard.
Make a thunder tube. (Recommend not using this with infants or children who are easily startled.)
Explore the sounds of rain
Take an umbrella or a plastic bag over a container outside along with a bowl of water. Use your fingers to sprinkle the water on the umbrella/bag to create the sound of rain.
Check out videos with the sounds of rain.
Rain down a song
Some of my favorite rain songs to sing with children are:
Rain down a book
Sharing books is important. Here are a couple of my favorite rain books (affiliate links).
Share Rain Train – a singable book.