We usually travel with our friends, and we each have two kids. We have surprised them twice - but before we were actually leaving to go on the trip. First time was 2 weeks before, and to me, that was the perfect timeframe. It gave them a little time to get excited about the trip, but not too much time for them to ask "How much longer?" Mine also like to pack certain books, stuffed animals, etc. so it just worked better to tell them a little in advance. But, I would LOVE to pull one of those "Hop in the car, we're going to Disney World!" surprises...maybe next time!
First time, we did something sorta like a scavenger hunt. The "prizes" at each stop on the hunt were sealed envelopes with clues inside. Since the youngest couldn't quite read yet, they ended up with three envelopes with clues, which the three older kids read out loud. Then the last clue had the youngest go find a "special box" hidden in the house. Inside the box we had Mickey cookies and some Disney brochures. It took them a few seconds to catch on, but when they did, they were so excited!
For our trip in 2 weeks, we surprised them on Christmas Eve. (yes, it has been a LONG 5 months but they have had a great time doing a daily countdown!) They each got a large Christmas box, and we had them open them at the same time. Inside were disposable cameras, Mickey fruit snacks, the Birnbaum book for kids, and all kinds of Disney stuff. Needless to say, they thought it was the best Christmas present ever!