I never plan a durn thing. I don't even make ADRs, unless I'm planning a meet or a special occasion.
Going with the flow can be terrific, but it only works if you know WDW well enough to avoid the typical problems. Knowing the average crowd patterns, which rides are most or least popular, and the times of day or days of the week when each park has it's heaviest or lightest crowds, are all vital knowledge to decide where to go and what to do.
It's also helpful if you can be flexible. If a restaurant you like is all booked up for the evening, you have to be able to move on and come back later. Getting in at off-peak times can also help.
Many times I have gotten to a park in the morning only to find it mobbed. But being flexible, I simply moved on to another, less crowded, park. Park hopper passes come in real handy for this.
Flexibility and spontinaety are great, but the best way to enjoy them is to be forewarned and forearmed.