Are you driving or flying?
A few years back I surprised my son with a trip to Disney. How did I do it? We frequently talk about Disney and the things we like and want to do so I didn't even tell him we were going anytime soon or within the following year. My son lives with his mother but I happen to have him for the week for his mom was going to be out of town with my daughter. The time frame was May and he was still in school. I told him that I would take him out of school for one day to go to Kings Island (which is about an hour away). So on Tuesday night, I told him on Wednesday I was taking him to Kings Island. That night my wife and I packed his clothes and items he would need for the 5 day trip and placed it in the truck of the car. That morning we got up early and drove to the airport (he was thinking we were going to Kings Island). The airport was about an hour away so that worked well for my plans. My wife went with us (which worked well into the surprise). When we got to the airport, my son thought my wife was going somewhere. But when I told him he need ed to get out of the car, he said, "what?, why?" I said, "We are going to Disney World!" He was speechless with his mouth wide open. Then he flew out of the car and he was ready to go. He was so excited. To add to the story: As most of you Disney Lovers, we are big planners and plan our trips months if not years in advanced. I planned this trip on Sunday and we left on Wednesday. In all the 20+ trips to Disney, this was one of the best ones ever.
I hope this might give you some ideas on your surprise planning. Have fun!