We did one surprise trip, partly because we didn't book it until 5 weeks out anyway, so it wasn't such a long time to keep a secret. Otherwise, it is more fun to be able to plan and talk together.
Anyway, we did it at 10 days out, which the girls said was perfect. They could still make a countdown chain, pack for themselves, plan a little, and prepare to miss a little school (3rd and 5th grade). I feel springing it on them day-of would have caused some anxiety.
We prepared a treasure hunt in which at each location there was a math problem for each of them. The answers each corresponded to a letter, and each of those clues were used to solve a code phrase at the end, that said "Look at the November calendar". When they flipped the calendar from October to November, they saw Disney World written on our dates. They screamed and laughed and fell on the floor together, so it was a success.
