We surprised my kids on my DD's 7th bday (well, the surprise was mostly for DD as DS was 20 months and didn't really know what was going on). It was both of the kids' as well as DH's first trip. I had gone out and bought her several gifts that I had hoped would be clues that we were going to Disney, as well as be useful things for the trip. I found an American Tourister suitcase that had a fireworks design on it that said on the tag "Official Luggage of Walt Disney World." Inside, I had wrapped Disney themed gifts like a character bathing suit, sunglasses, Disney World shirt, photo album and frame, autograph book, Disney guidebook/ journal for kids, old school Disney World Barbie, planning DVD, map, and calendar with our dates circled. Other relatives gave her Disney gift cards. She liked it all but was pretty oblivious.
Meanwhile, we had snuck a present onto the porch and had someone knock on the door. She went out and found a card with Minnie on it saying "Happy Birthday! See ya real soon! Love, Mickey, Minnie, and the gang." There was also a small plush vanilla-scented cupcake with a mini tsumtsum attached to it. She thought it was neat that the characters had sent her a present, but still didn't really understand. Finally, I gave her a postcard that a lovely lady here on the boards had sent her from Disney from Mickey and the gang. It said, "We can't wait to meet you on your first trip to Disney World! Love, Mickey, Minnie (& the rest)" She read it, looked confused, read it again, then almost whispered, "We're going to Disney World?" Then yelled, "Oh my gosh, we're going to Disney World!" It was great! When we went to Disney, she even thanked Mickey for his gift and the bday card and postcard. He of course went right along with it, putting his hand over his heart like he was touched she remembered and giving her a huge hug. It totally helped to reinforce that magic for her.