Hmmm. Excellent question.
I am the trip planner in our household, but I do it in cooperation with my other half. So when we travel - except for our first trip to Europe - it's always a joint decision about destination(s). I am still the one that books the hotels, looks for things to do, books walking tours in advance, etc., etc., etc.
So my trip planning varies, based on the following three factors :
-- Destination
-- Purpose of trip
-- Have we been to this general area before
For example, I'm currently prepping for my 5th trip to Italy. We will visit a different part of Italy than we've previously traveled. To go to Italy, I do a ton of research. I will book or have planned at least one major thing to do/see/experience on each day of our holiday. We won't use most of my research, but it comes in handy after we've done that one major thing because we don't go to Italy to sit in the hotel room and watch television all day.
Sometimes we plan trips to a beach here in the US. Those trips are generally for the purpose of walking the beach, looking for sea shells, checking out the local cuisine & checking out the local history/sites to see. I also plan things for those trips, but I plan them on a much more relaxed basis. I take advantage of seeing local historical sites, botanical gardens, animal rescue sites, etc. These trips are far more easy going, but I have done enough research ahead of time that if we decide to change gears and go-go-go, then we're able to do so. Same thing if we're doing a US city holiday. I plan a major must do thing, and then everything else is a bonus. That allows us the flexibility to change gears if we realize we'd rather do this than that.
BUT - and this for me is an important but - I plan trips differently now than I once did. At one point in life when our vacation time/funds were more severely limited, I planned to fit as many things as possible into one day. Now that we no longer have to work within the same constraints, I try to deliberately work in some time to do nothing more than get lost, explore and experience certain places which we visit. (The get lost part wasn't on purpose, but once I realized that I ALWAYS get lost when I travel, I gave into it. We've discovered some fabulous restaurants that way.)
Long story short - I plan ahead. A lot. And then I judiciously use what I've learned. But we always have some key outings scheduled, even if only in mind. We like to see/do. And the planning ahead allows us to see/do a lot or a little, based on how we feel that day.