Rainy days evoke different emotions in different people. Depending on the personality, one person could feel depressed while another energized. Pick one of your favorite characters to write about, and give them a rainy day to live through. See what they do. Maybe they'll surprise you.
What do you think of when you hear rain? What emotions do you feel? What about your characters, what kind of emotions would they feel?
For this exercise pick a character and setting, let it rain, and see what comes out. It can be of any length, though is sh0uld be reasonable. Don't write a full novel and don't write just one sentence. For now, stick with prose instead of poetry. You can do this as many times as you want for as many characters.
If you're confused, the second chapter is mine and should give you a pretty good example of what I'm talking about.
The most important rule, is enjoy yourself. You may learn something about your character that you never new.