AIBU to think this is a horrible sentence to hear? Does anyone actually like surprises?
It is one thing when you have a birthday coming up and someone says "do you want to know what I've got you or would you rather have a surprise?" (Fine, it isnt a real surprise, I know youve got me a "thing"). But does anyone in the world appreciate a random "I have a surprise for you"? This could mean anything from "I got you a book" to "I'm coming to visit and stay with you for a month".
YABU - everyone loves not knowing the things that other people know and having no control over the thing that has been planned and about to happen.
YANBU - tell me what it is so I can plan my day/time/meals/outfit/finances/purchases accordingly and think about my reaction ahead of time.