Word of warning! I once orchestrated a surprise trip for my kindergartener. I was a single mom at the time. I pulled my son out of bed early in the morning and didn't do the big reveal until we were at the airport. He seemed happy, but unsure. While we did have a good time on the trip, it was marred by by son's pervasive uneasiness.

He kept wondering what was going on at school (I had pulled him out in September) and repeatedly asked why Grammy (my Mom) hadn't come with us. He worried about our dog. He seemed *very* happy to go home. (Not my intended/desired reaction).
I learned quite a lot from this. Children, even small ones, have routines and obligations. Maybe your young child will worry about missed schoolwork, soccer practice, or just want to clear something up with a friend on the playground before leaving.
Your child may feel more in control if he/she can pack his/her own special belongings and say goodbye to family and classmates. Older children can read about the parks and help plan the trip tailored to their quirky interests. What they want to do/skip may surprise you.
Finally, there is the power of anticipation! Christmas morning is special in part because of the build-up (decorating, carols, baking, cards, etc.) We count down to it, plan for it. I don't know if it would be as cool if a bunch of presents just arrived unexpectedly in the middle of May.
So while, I think it's a fab idea to surprise kids creatively, I think it's best done a month before the the trip. Long enough to prepare and build anticipation, but short enough, that it's not unbearably long.
