Why are you so set on SURPRISING her, when you know going in that she's already NOT pre-disposed to being excited? You could do a whole bunch of lead-up, where she's having fun with the treasure hunt or whatever, and then she finds that her "prize" is a trip to Disney World ... something SHE DOES NOT WANT. It's like your husband making you follow clues on your anniversary, and you follow these clues all the way through the house, getting closer and closer to the "surprise," only to discover that the "big gift" is a new ironing center. What a letdown!
Instead of surprising her, why not just tell her that you're planning a vacation to WDW and you plan on taking her along and you'd appreciate her help in the planning. If she's playing the "I'm too cool for Disney World" card, pretending to be more mature or whatever, then play into that. Sit down and make a list of her favorite things, and then talk to her about how those things translate to Disney. Does she love animals? You've got DAK. Is she a beach person? You can rent Sea-Rays. Is ice cream her favorite food? Beachs & Cream. You get the idea. Tell her to write down everything she really really likes, and then tell her that you bet that you can find something at WDW to match every single one of her favorite things. And then do it.
Something like that -- where the planning is part of the process -- will help her feel more grown-up and allow her to change her mind if she wants, but to not have to admit that she's all of a sudden interested in Disney. She can still say, "Well ... I really hate all the Disney cartoons," but she can add on, "but this Animal Kingdom place sounds pretty cool" and not feel like she was snowed into something.
If you just HAVE to suprise her, save that for the actual trip. She's a beach kid? Great ... one day during the vacation, surprise her with a day at a water park, dinner at Beaches & Cream, and a surfing lesson. Call it "The Day We Wear Swimsuits All Day" or something equally silly, and make it special. But truly ... surprising someone with something they don't want will be a let-down for you both!