What a wonderful surprise!
A couple of friends did this with their young son a few years ago. They had booked a cruise for later in the year (that their son knew about), and one morning told their son they'd like to drive into town to show him what a different cruise ship looked like. (The later-in-the-year cruise was to be their son's first.) Once they got there, they kept it going by saying, "Maybe they'll let us go inside." Of course, they were booked on this cruise, as well.
Being practical here -- just remember to bring your wife's passport or birth certificate, and make sure she takes her drivers license with her. I'd hate for this to fall apart last-minute.
Have a WONDERFUL time, and let us know how it goes!