Do you have a smart phone? If you do, I highly recommend getting a weather app that includes radar. I had that the last time I was in DLR when it was rainy and it was very helpful. We could look at the map and gauge when there will be breaks in the storm. We used those breaks to go from place-to-place and we also could tell when heavy rain was coming and we knew not to head to an outdoor queue at that point!
Rainy days are good for shows (e.g., Aladdin, Playhouse Disney). Attraction lines tend to be shorter than normal (because folks stay home), so it's a good chance to get in rides. However, keep an eye on the forecast and avoid the outdoor queues if it's going to pour.
Also note that some attractions will shut down if it's raining. Some will only close if it's pouring, others will close when it starts raining. Most any outdoor ride will close if it's rains hard enough and long enough.