"Must-do"s depend very much on what you and your girlfriend are interested in. Do you like thrill rides? Scenic jaunts? Pool time or park time?
As a very generic plan, I'd suggest the following for your 8 days:
Day 1 - Travel / check in to your resort / Visit Disney Springs (formerly Downtown Disney) for a nice table-service dinner and shopping or bowling. If you have more than half of a day available, go to the Magic Kingdom.
Days 2 - 4, 6 and 7: Park days. Spend at least 1 1/2 - 2 days in the MK (must-see: Pirates of the Caribbean, Haunted Mansion, Splash Mountain, Big Thunder Mountain, Peter Pan, It's a Small World, Dumbo, Mickey's Philharmagic, PeopleMover, WDW Railroad), 1 - 1/2 days at Epcot (must-see: Soarin', Test Track, Mission:Space, Spaceship Earth, a walk around World Showcase), 1 day at AK (must-see: Festival of the Lion King and/or Nemo show, Expedition Everest, Dinosaur) and 1 day at HS (must-see: Great Movie Ride, Star Tours, Muppets 3D, Rock 'n' Rollercoaster, Tower of Terror, Toy Story Midway Mania, possibly Beauty and the Beast or the Indiana Jones Stunt show, Fantasmic). On day 5, don't plan on visiting a park. Sleep in, go swimming, and consider mini-golfing at Fantasia Gardens or Winter Summerland. Alternatively, consider going to a dinner show (e.g., Hoop-de-Doo Revue). Plan to see the MK fireworks (a definite must-do) on your last night, and ride the monorail somewhere (another must-do) at least once. If your budget allows, have a leisurely breakfast in your room, a CS lunch, and a TS dinner each day (if you don't have TS reservations already, MAKE THEM NOW). If you can, get to a park for rope drop every day. If you can't, plan carefully and utilize evening "Extra Magic Hours" whenever they are available. Be aware in planning your days that attendance will be higher at whatever parks have EMH on a given day, and if you have parkhoppers and want to take advantage of morning or evening EMH, hopping to/from a less crowded park may be to your advantage. Every day after lunch, go back to your resort for 2-4 hours to escape the worst of the crowds and heat, grab a nap, take a swim, and refresh and recharge yourself for the evening.
Day 8: Revisit your favorite attractions at your favorite park / travel home.