If your child is expecting to do one thing tomorrow but you have to tell them you're going somewhere tomorrow (so tell them early evening today rather than tomorrow morning). It's somewhere really good that they will enjoy so it's not that that's the problem. But in their head they don't think they'll enjoy it and I believe this is just because they weren't expecting to go.
Do you do this or do you just never change your plans?
Is there any way to tell them to make it easier on them?