I vote for Outlandish reveals, always a good thing! But it's good that you're giving him a couple of days notice too. Looking forward to a trip is half the fun!
How about some kind of a scavenger hunt around the house? You could give him a bunch of little gifts for the trip that include a clues about where to find next gift. Write something on the tag like: "Look someplace hot and dry for your next present" (in the dryer).
Start vague, with something like a luggage tag and then work your way up to more obvious things like sunblock and snorkels. The more you can build the suspense the better. For fun I think I'd try to throw him off a few times too, with something really obscure (like with white vinegar... in case you get stung by a jellyfish of course!)
Or, you could just do regular little gifts like pieces of candy, but hide them with in things he'll need for the trip (suitcase, beach towels, swim suit).
Fun! Come back and tell us what you decide to do : )