I hate disappointing my kids. I avoid it if at all possible. I used to tell them: "Guess where we're going on Thursday?! We're going to the zoo with Aunt Jamie and your cousins!" Then one of her kids would get sick and I'd have to say: "Well we're not going after all, sorry!"
I hated doing that. Life will have plenty of disappointments and my kids will have lots of opportunities to develop flexibility (after all, they have 6 siblings!). I control what I can.
I give them 20 minutes notice for any exciting activity. I plan to take them to the zoo this morning and I can hardly wait to tell them. But I haven't told them yet. I'll tell them 20 minutes before we need to leave. ;)