Sounds like most of the time the surprise approach is a blast, but I agree with the advice to do this only if you're very certain your child would enjoy it. The uncertainty implied in your question makes me wonder if you have doubts.
My oldest is a routine-oriented kid, and really likes to have a sense of control. We put 'trick candles' on his birthday cake when he was 6 (figuring it'd be most fun to do that at an age when he hadn't heard about them yet), and to say that he didn't enjoy it would be an understatement. I have video of a very worried looking 6 year old and for 2+ years we dealt with irrational fire phobias. O.K.....so this is a stretch in terms of similarity to a vacation surprise, but I thought I'd throw it out for what it's worth. I certainly plan some little surprises.....afterall, part of growing up is learning to deal with surprises either good or bad.....but I'd give some serious thought to whether I wanted to spring anything major on him as I've learned he might not have the reaction I expected.
Again, it's probably just a small % of kids who wouldn't love the surprise, but I wouldn't risk it if you're concerned even if it is just a minor risk.
Have a great trip!