My degree of planning depended on how many days we would have at the parks. Anything less than 7 days meant we would have a highly structured plan that was rarely deviated from in the morning hours. The Type-A drive relaxed after lunch to cope with crowds, heat and exhaustion. There still was a plan, but it wasn't necessarily followed to the letter.
I think the key to our success, though, was much more about adapting to the needs of our children. There were days when we had several must-do items still on the list when it was apparent that our children (or maybe the adults!) were not going to be able to make it. It was our duty as parents to adjust to that because no degree of Mickey Magic was going to be able to make the afternoon/evening enjoyable without a break. Sometimes it simply meant we needed to remove the over-stimulation of Disney and head to places like Tom Sawyer Island or the Bug's Life Playground for an hour or more, but it normally meant we needed to vacate the parks completely for a well-deserved nap and ramped-down dinner. The nighttime events were and are way too much fun to not have everyone near their peak attention/excitement/energy levels.
We either do a walk up or call guest services in the morning and ask what is available for the park we are at that day. I know, it's a little risky, but hey, every day should be an adventure in fun! Unplanned days are some of the most memorable for us! 
