Uh oh. How does she handle disappointment? I know I would be if I was looking forward to something to find out later it's not happening. Maybe you reveal the cruise at the same time as you "cancel" the trip to soften the blow?
My kids can never hold back surprises from me. They're too exited. Anyway, I ask them not to tell me what it is, but instead to give me clues. Makes it more fun for all of us. You could do let her know the trip in Feb is just a decoy and to figure out the real vacation she has to solve a series of clues. Have something like a golden gate bridge, a cruise ship, and some salmon as the clues. Maybe even wedding rings or something so she figures the date too.