Well, since so many people replied to my OP, I thought I'd update everyone on how our Xmas suprise went. We decided to go with the puzzle idea and made up a graphic that had some pictures on it like fireworks, plane, etc, and then we wrote "We're going to Disney World" on it, but spread the words around, used differnt fonts and colors even in the same word so it would be hard to decipher until it was all together. We also purposely didn't use any Disney characters or logos because we didn't want them to be able to figure it with only a few key pieces put together. We then uploaded it to Walgreens website and ordered a 8x10 puzzle. We were pushing it a little on the timing, but thought we'd get it before Xmas. Well, of course we didn't! So Xmas Eve, we're trying to figure out what to do. Then I recalled seeing websites where you can upload a pic and instantly make an online puzzle that you put together on the screen. We found a site and did that, then brought it up after the kids opened all thier other gifts and told them they had to do the puzzle to get thier last present. It took them about 5-10 mins and had to look at it for a min when it was done to figure it out. They were surprised, but nothing over the top (no jumping up and down, screaming, etc), but I think it just took some time to sink in. Since then, I think they have been getting more excited about it as we've been talking about it, planning meals, etc.
Oh, and even though I was pretty PO'd that we didn't get the real puzzle in time, it probably turned out for the best. When we did finally get on the 27th, it took my wife and younger son about 45 mins to put it together! If the 2 boys were doing it themselves Xmas morning, it probably would have taken an hour or so and they would have been annoyed with the whole thing!
I only have a few posts here, so I can insert a link to the online puzzle. After I make a few more posts, I'll put the link here if anyone wants to see it.