What a great idea to surprise your kids! Here's a few ideas for gifts that won't give away the surprise-
How about a pirate hat, eye patch, etc. (if he's into pirates)? Or something he will use at the parks-like a fanny pack just his size, new sneakers (to break in before you go!), a mister/fan/water bottle (you may not need this in Feb., but it really keeps my daughter entertained in Disney lines!), action figures/hand held game to play with while waiting in line, or new swim suit, goggles, life vest, beach towel or floaties for the pool? Or maybe an empty scrap book/box to collect and hold memories, or an inexpensive camera/underwater disposable camera? Or a tube of quarters and shiny new pennies for collecting pressed pennies around the world. Or a bunch of glow in the dark bracelets or light up toy for the night time parades. You could make his packing list and then see if their is some thing he might need for the trip- like a kid sized rolling suitcase, backpack, or games/activities/art activities/snacks for the airplane.
Good luck with the surprise- and have a magical trip!
Roo