Epcot is definitely a whole day park. Depending on what you family enjoys doing, some "must-sees" would include:
Spaceship Earth > me, DW, and kids -8 and 5 love it; some say its a bit tedious
Test Track > would not miss this one; a great ride
JIYI w/ Figmnet > especially for the younger ones
El Rio del Tiempo > sort of a "Small World lite", w/ a Mexican theme of course
Illuminations > I think me and my family are the only people on the planet who thinki this is kind of boring, but everyone else loves it. So.......
Universe of Energy > long but fun
Living Seas > Turtle Talk with Crush was a huge hit with my two young ones
Soarin' > getting ready to open; supposed to be unique and special
strolling the World Showcase > kids may get bored, but there is lots of neat stuff at each pavilion
American Adventure > I have never seen this one

but it is said to be really good
Animal Kingdom: NOT a zoo... and NOT a who day park
Kilimanjaro Safaris > signature attraction; must see
Its Tough To be a Bug > nicely done 3-D show but sort of creepy if you are scared of bugs
Festival of the Lion King show > worth seeing once; nice show
Dinosaur > some say its great, but I remain unimpressed
NOthing else I would go out of my way to see at AK
Hope this helps; have a great trip with lots of peace, love, and pixie dust