As Alex said above I have a bucket with rice, lentils, tiny pasta stars/ABCs, dried beans, etc and little jars, yogurt pots, plastic spoons, funnels, etc and I spread it out on a bedsheet on the living room floor and we scoop and dump, drive cars through it, etc etc.
We watch a lot of TV - I'm not against it and that's what rainy days are for! I let him watch his normal shows (Playhouse Disney) on the laptop which is a huge treat for him for some reason, even though it's the same stuff as on the big TV!
I pull a chair up to the kitchen sink and put plastic pots, spoons, etc in and let him play with water.
We have a couple of toys that only come out on rainy days - a motorcycle racetrack that you use remote controls to drive them, a tent/tunnel, some blocks, and so on - nothing special about them just that we save them for rainy days.
We do housecleaning when we are stuck in - we are out from wake up until bed on sunny days and almost as long on nice enough days so when rain strikes it is a chance to catch up on housework and DS helps me out with that. I give him an wold Windex bottle with water and vinegar in it and a drop of green food colour so he thinks it's the same as mine

and a roll of paper towels and he squirts the kitchen to death and dries it all off - saves me a job!!!