First off...
I have some ideas...but you might not like them. This Board is a dangerous place to come with a question such as yours. There are lots of people who post here who are obsessed, (in an almost clinical way), with WDW and allocate the majority of their vacation time to it. Some even vacation there exclusively.
"I can't imagine why anyone would want to go anywhere else!" So when folks like that weigh in, they are going to try to convince you that there are lots of things to do with your fiance that she will find enjoyable and that you will eventually be able to convert her. And I won't dismiss those options entirely, as WDW is extraordinarily diverse and offers lots of different ways to have fun. But there are some things that it simply does not have and never will. If your fiance loves to ski, or scuba dive, or go to art museums, or visit wineries, or mountain climb, or if her idea of vacation is one Michelin-starred restaurant after the next, or if she likes to immerse herself in foreign cultures, or volunteer in developing nations, then no, there is no converting her. WDW will be way too frivolous a place for her. For many people, vacations are all about connecting with the real world. And WDW is the ultimate escape from the real world, and many people go there specifically for that reason. Neither view is wrong. But they are largely incompatible.
But that doesn't mean that she can't or find a few things there that interest her, once in a while. And let me emphasize that. ONCE IN A WHILE. You have to accept the fact that she may never be a "
I want to go twice a year, every year" person. She might never become a "
I want to go once every other year" person. If you are going to enjoy a long and happy life with her, you are going to have to compromise. You are going to have to find enjoyment doing the things that she likes to do and vice versa. So in your quest to mesh your love for WDW with her disdain for it, try not to go into this with the idea that you can, or should, convert her into being a super-fan. Try baby steps and try to convince her that there are things that you two can do together that are really fun and that she will enjoy, while at the same time embracing the things that she enjoys, even if they are out of your vacation comfort zone. But don't try to "win" this battle by turning her into a DVC member who wants to go to WDW all the time and ride all the headliners. While anything is possible, don't bet on that outcome. It may be that the things she enjoys most about WDW are hours on the Pangani Forest Trail. Or a spa treatment at the Grand Floridian. You have to face the reality that life with her will not be lived with Mickey ears on at all times. If you aren't OK with that, then "Thar be rough waters ahead!"
Specific answers to the "what should I do" question are a bit difficult until we know what she enjoys doing day in and day out, and how she likes to spend her vacations. Give us some examples of "if she had her way, her dream vacations would include ___________" Once we know how you would fill in that blank, we can better assist in making recommendations of things to do in or around WDW that would appeal to her and might make her think twice about having to be dragged there.