My favorite park is EPCOT, so I like to start there. But if you're staying on Disney property, see if any parks have early entry, and open up that park. If you're not staying on Disney property, see what park has early entry, and DON'T go there that day.
Magic Kingdom all the way. Check out the nightly offerings. You've got Main Street Electrical Parade, Fireworks, the Little Mermaid ride at night is amazing, when you leave the park you may catch the water electrical parade out on the water, visit the Grand Floridian for a late night bite at their resturants or bars or check out the Polynesian Hotel at night, all amazing and in the same area.
Oh wait, you posted this back on May 1st. I'm assuming your trip is probably already over. This idea of mine will also work on your final night, whenever that is.