My favorite park, by far...mostly due to World Showcase, which is my single favorite thing about all of WDW.
World Showcase
Mexico: Make sure you go inside the temple and check out the "marketplace". Some very cool stuff they sell. Then take the boat ride. Also, if you drink, do not miss their margarita stand outside the temple. Ohhhh boy...good stuff.
Norway: Maelstrom is a lot of fun. Gotta try that. And the Puffin's Roost is also a great store for souvenirs.
China: Pu Yang Acrobats. They're awesome. Surprisingly, the "Wonders of China: Land of Beauty, Land of Time" 360 degree movie they show is also very cool. One surprising hidden gem: the Lotus Blossom Cafe. Try their red bean or ginger ice cream - it doesn't sound it, but they are both DELICIOUS.
Germany: Biergarten is VERY good. Buffet of incredible German food with an Oompah band who plays at certain times. It's fun. Much cheaper for lunch though, and the menu is pretty much the same. Highly recommended if you have an appetite.
Italy: I don't usually spend much time here; not a whole lot to do other than admire the scenery, which is beautiful. Awesome architectural work.
American Adventure: Make sure to check out The American Adventure show, as has been suggested. It is very cool. And hopefully you can catch the revolutionary-era flute and drum band that marches around outside at certain times.
Japan: Possibly my favorite country in the showcase. The Koi fish pond and garden is very relaxing and mellow. The Japanese taiko drumming is cool too. The Teppanyaki Dining Room is DELICIOUS - highly recommended (and it's balcony is a GREAT place to watch illuminations at the end of the night). The Mitsukoshi Department Store has some great stuff in there - another great place for souvenirs.
Morocco: Don't spend much time here, but both the Tangerine Cafe (cheaper) and theRestaurant Marrakesh (more expensive) are both VERY good. The band that plays out front is also very entertaining.
France: Pastry shop. Wow. Also, their 360 degree movie, Impressions de France is entertaining.
UK: Check out British Invasion, a Beatles cover band. They play out back. Their shops are excellent. I've never eaten at the Rose & Crown, but the Fish & Chip shop is very good (if you're into deep-fried fish and fries).
Canada: Another 360 degree film that is worth catching if you have time. Absolutely beautiful. The waterfalls out back are nice. And make sure you check out "Off Kilter", and unique rock band who plays on the stage out front. They're awesome.
Make sure to take World Showcase at a slow pace and absorb everything - leave plenty of time for it. It's particuarly awesome at night. And Illuminations is a MUST SEE. Don't miss it.
Future World:
Test track: MUST visit. It's a thrill ride, but it's not as intense as Mission Space. But still intense enough for those who like thrill rides for the thrill.
Soarin: MUST visit. Remember to inhale through your nose every time a scene changes on the screen...they pump in scents to match the scenery.
Mission Space: Only if you have a strong stomach. It's a tough one. I enjoy it, but I admit it gets me a little queasy.
SpaceShip Earth, as has been said: classic ride and still interesting. Very relaxing.
Innoventions: some cool stuff here. Can take pictures of yourselves and email them - for free.
Ice Station Cool: hopefully it will be open by the time you go. It's being rennovated at the moment (anyone know a time-table for this?)
Fountains: Take some time to watch these move to the music. Very cool place to sit down with a drink or a snack and appreciate it all.