I go with a general plan. I only freak out if I feel my plans are being hijacked and I'm not in control of my situation. Such situations usually happen when I'm with my parents on vacation. I have to rely on them for everything when I'm on vacation. They do the driving, and I'm subject to their whims. I have since refused to go on vacation with them.
However, today I made plans with a friend to go walking on the local bike path. However, he wound up bringing his friend along, and we spent little time walking on the bike path and more time at the shopping center nearby. This was fine with me since:
1. My friend's friend and I totally hit it off.
2. It was a hot day, and the stores have air conditioning.
Years ago, I might have not dealt with this so well.
However, when my parents would say we were going to visit my cousins, and then decide to spend the rest of the day with my senile grandparents, you have no idea how close I would come to a meltdown. Actually, being dragged places by my parents drives me insane, because they just plan as they go, and when I don't know what I'm doing and I can't leave, that is what pushes me over the edge. 
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"I'm sorry, I seem to have a tin ear for other people's feelings..." -Naoto Shirogane