A few years back, we went on a rainy day and were able to ride Splash Mtn. 9 times. It was awesome. That's not the kind of ride people want to go on when it's raining, but it's my favorite, so it was awesome for me.
Yes the rare (lately anyway) rainy day at DL is awesome, I just love the rain and it is usually not very crowded, you get to see the park from a whole different perspective which is cool. What better way to spend a dreary day then at the happiest place on Earth