I have surprised my daughter, now 10, 3 times and am working on a 4th. I also surprised some of her friends with a trip! I started doing the surprise trips so she didn't go on and on at school about going to WDW for the ump-teenth time!
1st time (she was 8) I packed without here knowing and picked her up at school and took off on the 14+ hour drive. Only about an hour from home she started to panick about where we were going so I played the video of WDW-Building the Dream. When she figured it out she got so excited and started crying....it was great! Fun 1st trip during Halloween!
2nd time- Told her I had a meeting in Gainsville....which she believed. Told her she could go but it was to be a quick trip with lots of driving. She wanted to go, thankfully! As we got closer, I called a friend and he said the meeting was canceled....Oh no. Now what do we do? She was crushed since we had been driving for 12 hours! Well, since we were close to the Disney welcome center we decided to go there. Hey, while we are there lets call my parents who are...surprise...in Orlando in their camper! We met them at ASM and then told her we were going to be at POR for a week! Worked out great!
3rd time- Invited a friend of hers to Fla. Told my daughter we were going to Wild Adventures in Valdosta Ga. Got there and decided to go to Ocala to eat- long drive just to eat! But at the Disney welcome center I surprised BOTH of them with a Disney adventure instead! They started screaming and jumping up and down. It was her friends 1st WDW trip! 1st trip to PC, which we loved!
With friends: She knew about the trip but they didn't. The story was, they were going to Canada. They came to say goodbye to us and invited my daughter. She 'wanted' to go...even started 'packing' another suitcase (the van was already packed) so then the other kids invited me too. We played a game to see if I could go! So we are all off to 'Canada'. After driving all night my daughter woke up just as we got to the Welcome to Disney World sign (guess she could smell it!!!) at 5 a.m. We woke the other girls and they all screamed and screamed!

The oldest asked if it was planned after we checked into the CR for the week.
I am working on another but its hard not to tell her. She told me all she wants for Christmas is a Halloween trip to WDW!!!
Like someone else said...surprises are great if they work out. The WOW factor really makes it wonderful. My daughter loves surprises!