Hi Kim!
Don't worry about a rainy day at Walt Disney World. In Florida, the humidity can cause a chance for rain everyday, but it is usually just a brief shower.
If you do end up touring a park during a day of steady rain, your best bet is
Disney's Hollywood Studios (HS) because most of the attractions at HS are long, indoor shows. During our last visit it rained the entire day during our visit to HS, but it didn't affect our fun at all! We were able to enjoy Beauty and the Beast Live on Stage, Toy Story Mania, Voyage of Little Mermaid, Rock 'n' Roller Coaster and more without getting soaked.
But, armed with a poncho, umbrella and a hat, you can enjoy any of the parks with a drizzle.
I use touring plans dot com to help me understand crowd levels and deciding which park to visit on which date. For a small fee, this site will give you the crowd levels, based on a scale of 1-10 for each day, recommendation on best park to visit and touring plans based on your needs. It is well worth the cost in my opinion!
In general, on weekends, when it is more crowded, it's best to visit Epcot because it is so spread out that it doesn't feel crowded. Animal Kingdom also tends to be less crowded on the weekends. Magic Kingdom and Disney's Hollywood Studios are best during the weekdays.
Please let me know if I can help you with anything else at all!
Magically,
Stacy