Agreed. You really can't go wrong here. I've taken DD several times over the past few years. Sometimes she knew, sometimes it was a surprise. Both ways are great.
My thing now is I need more creative ways to make the secret work if I try that again. I've done:
"We're going to visit Grammie" and then pulled up at the Disney hotel saying "Oh, yeah, we're also going to Disney."
I used dear Grammie one other time and told DD about the WDW part by pulling out some Disney stuff on the plane.
"You know that restaurant at the airport you like so much? I thought we'd go there for dinner." and the pulling the suitcase out of the car once we'd parked. (She was younger and really did like the restaurant - one of the waitresses remembers her every time and talks to her a lot.)
When I picked her up after school, I told her I'd have a really rough day and just wanted to drive around a bit. We 'just happened' to wind up at the airport. (Works if your airport is close like ours)
I really thought our 1st trip would be our only trip. Little did I know. <grin> For our secnd trip, I told DD that a friend of mine wanted to take her DD to WDW and wanted to know what a kid her age would really like to do. In that great kid-scrawl, DD made out a list of everything she had really loved doing. When it got close to time for the trip, I asked if she remembered it and how would she like going back to do all those things.
Upshot is that planning with the kids is fun and surprising the kids is fun. I admit that, while I do plan most trips with her, I love the look of surprise and sheer delight she gets when I surprise her.
