Stay in bed, grab all the pillows, get comfy and read.
Go through your closet and make a pile of any unwanted items to give to charity. The same can go for your bookshelf, children's clothes and toys.
Bake. Cookies and cake are great on a rainy day.
Take photos of the clouds, puddles or anything that reminds you of rain.
Ring a friend and talk for an hour (as long as it isn't storming).
Rent a movie you loved as a teenager but haven't seen in years.
Write a letter to a friend and tell them a story about how you met, or a funny memory you have of a time you spent together.
Make yourself a mixtape, whether it be on your MP3 or burnt to CD, go through your favorite songs and make the perfect album for driving to work, or gardening, or just for a rainy day.
Take up that hem and sew on that button that you have been meaning to do for ages.
Sit in your favorite chair and knit or crochet or (insert craft of choice here) , listen to the rain and let your thoughts wander.
Do you have any rainy day ideas to add to the list?