I'm looking for a way to surprise our kids, and the puzzle idea intrigues me. DH & I have talked about a puzzle idea, but would be with an existing puzzle. I would put it together, and write a message on the back. "We're going to Disney World" or something like that. Then I'd take it all apart again. We'd put the puzzle together as a family, then either in the process "discover" that there are markings on the back or somehow after it's done. Flip it over & see the message. We do puzzles in the winter a lot, and this would be our 3rd since December. We usually do them over the course of a week.
Possibly the bigger question, though... how in the world did you pack everyone's things & get everything ready to leave without the kids discovering it? We have considered this, but I always have to drag suitcases out, take the dog to the kennel, etc. and am afraid we couldn't pull off a "last-minute" surprise.
Last year on Christmas morning, we surprised the kids with a trip to Universal. We weren't going for 2 weeks, but my kids LOVE the anticipation of a trip and it was actually part of their Christmas gift. I wrapped 10 items in small boxes, numbered, and put them in a large box. As they opened each numbered box in order, it was another clue about the trip - Hogwart's t-shirts, a wand, a Sorting Hat Christmas ornament, leading up to mock plane tickets, mock Universal tickets, and an info sheet about our hotel with pictures. I'm just afraid that since we've done something like that before, they'll figure out anything we do SUPER fast.