A few tips from someone who has been going to WDW since her second birthday(me).
1. If you think a 23 year old with tatoos would be the most out of place person on the Dumbo ride, you are sadly mistaken.
2. If you are taking the Dumbo ride as a standard for other rides, you are even more mistaken.
If you are into more exciting rides, there are lots. Try Splash Mountain(especially if it's a particularily hot day), Space Mountain, The Haunted Mansion and Big Thunder Mountain Railroad in MK. You might want to skip the afternoon parade and go back for some more rides, but then again, I have seen the parades too many times myself, and that may habe some effect on that. If you don't mind rides that are a bit more timid, try Pirates of the Carribbean, The Tiki Room, and The Tomorrowland Transit Authority(which some classify as dull, but is a great way to relax after spending hours in MK, plus you get to see a lot of Tomorrowland, sitting down.) In Epcot, go for Soarin', Test Track, and maybe Honey, I Shrunk The Audience. If your up for Mission: SPACE, go for it!! But be warned, it should NOT be underestimated!!! In MGM, I'd suggest Rockin' Rollercoaster, Tower of Terror, Who Wants to be a Millionaire, Play It! and The Great Movie Ride. I haven't seen Lights, Motors, Action yet, but It's supposed to be pretty good too. Next is Animal Kingdom. Try Expedition Everest, Dinosaur, Kilomanjaro Safaris, Kali River Rapids, and It's Tough to Be a Bug.
Keep in mind that even the most expert WDW visiters find it near impossible to do EVERYTHING in a day. I suggest buying Birnbaum's Guide to WDW without Kids.
Last but not least, have a great trip. I hope its everything you and your fiance hoped it would be.