In general, after you've done the major attractions for the day, just take your time in each land exploring all the details. Many queues are extensively themed; don't rush through them. It's ok to walk slowly because unless the park just opened, you will catch up to a line anyways.
MK: If you like Beauty & the Beast, make sure to visit Gaston's Tavern and the nearby gift shop, even if you don't buy anything. Also be sure to have lunch at Be Our Guest (Dinner is served too, but is much more expensive, you may not get to choose where to sit, and you're not as free to just walk around and enjoy the scenery). The tavern has lots of details from the movie. If you're a fan of Dumbo, check out the Storybook Circus play area in front of Dumbo. If you're going before Halloween, look up on Main Street. There are carved "pumpkins" on top of the shops.
HS: There is a well to the left of the Indiana Jones entrance. Pull the rope. Don't skip One Man's Dream, especially because it may close to make room for the new Toy Story land. 50's Prime Time Cafe is a ton of fun to eat at, and if you're on a budget you can easily split the sampler meal; it's huge. Look in the displays at the front of the Chinese Theater.
Epcot: The mime in Italy and serveur amusant in France are funny. The Chinese acrobats are very talented. Don't miss the films in China, France, and especially Canada. Read about the rest of the entertainment in the World Showcase to see what sounds interesting to you. The fountain near front of the park also has water shows I think every half hour. Check out all the shops in the countries. There's many neat things to look at. France also has delicious pastries in the same building as the gift shop.
AK: On your way out of the park, explore the animal trails at the front.
