Congrats on your upcoming trip!
We surprised our kids by doing a treasure hunt through the house. We put numbered clues all around the house (I think there were about 6 clues) and then at the end their final task was to put all the clues in order and turn them over. Each clue card had a word on it which spelled out We Are Going To
Disneyland California! (we had to be specific as we live in Australia!)
My kids are twins - aged 6 and they absolutely loved this. We filmed it and they have very cute surprised faces when they read out the message!