I think there are more skipp-able things at Epcot than MUST-DO's. So I am going to give you my MUST-DO's at Epcot instead of my meh attractions.
Spaceship Earth - it's a classic, and it's a constant loader so the wait is never that long.
The Seas with Nemo and friends - I LOVE Nemo, and I think that ride is so clever! It's not like the other dark rides, and it's quick. Even if you end up not being crazy about it you won't lose more than 5 minutes of your day. And the aquarium upstairs is incredible!
Soarin' - aaahhh this is amazing. Love it.
Club Cool - AC, free drinks, and the amusement of watching people try Beverly. I go in here any time we are remotely close to passing it.
Test Track - it's just plain fun.
World Showcase - the rides/movies are fun if you have the time, but I don't think you'll feel like you missed out if you skip any of them. There are some stores and snacks that I feel no Disney trip is complete without...
Norway - stop at the Kringla Og bakery and split a School Bread or Almond Pretzel with your buddy. Mmmmmmmmmm.
China - the gift shop is just fun, some stuff you won't see anywhere else. And some of the stuff is surprisingly inexpensive!
Japan - the Mitsukoshi department store is wonderful! You can spend a mint on a kimono or $1 on Japanese candy. Make sure you browse around. I also always stop and pick an oyster and get a pearl! And get a Kaki Gori (kind of like a snow cone) on your way in.
Morocco - best counter service at Epcot IMO. Got a henna tattoo last time that was wicked neat, and it lasted for WEEKS!
UK - fun shops, get an Aero bar as a snack, and fish & chips!
OK - I think that's all for me. Whatever you do, make sure you have fun! Epcot is HUGE and a LOT of walking, more than most other parks, make sure you wear your comfiest shoes and bring a change of socks!