I totally feel for you. I had a big surprise for my DH planned this past summer and something completely random came up for the same weekend. I had to spill the beans completely to make sure he didn't make other plans.

I even had the plan on how I was going to tell him all worked out and...sigh... no fun for me or him (or so I thought...)
If it makes you feel any better, while my DS (obviously) wasn't surprised, he was very touched that I went through so much trouble (and he actually said that... out loud...on purpose... and this was coming from a guy who after more than a decade still blushes when he says "I love you").

He was also really excited for the weekend - it was actually kind of fun watching him try to play off looking forward to it.
I'm sorry the surprise was ruined, but I say go with it...bite the bullet, and present him with the whole plan soon - but maybe do something silly like print out a "travel folder", tie balloons to it or spill fake snow all over it and leave it all on his pillow tomorrow night.
Just because it's not a surprise doesn't mean it won't be a blast!
GL!