I love reading/hearing about people surprising their kids, but I'll never get to do it because my kids are the exact opposite!! My one is very much a "tell me exactly what we're doing along with plans B, C, & D" kind of kid, because he gets nervous easily. And my other is very much a "the journey is more than the destination" kind of kid. He takes everything as it comes and doesn't get nervous at all, but he loves the anticipation of anything. We often try to really slow special things down and really focus on them for him because he's the type to later say "Waiting for XYZ was more fun than the actual XYZ." We often do very spontaneous small things with them, so that the one can have fun without stressing over the unknown and the other can have fun without any journey to overshadow it.
I do, however, like to try to at least have some surprise for them during the trip....booking something special, or having a little something extra for them to use on the trip, etc.

That gives me a tiny bit of fun planning a "surprise" despite not having surprise loving offspring, LOL!