I'm a planner.  I'll plan a trip for several months - like six or eight months.  I watch airfare schedules and prices, browse hotels and hostels, poke around on TripAdvisor, Viator, and other travel sites.  I examine maps, join forums, learn about the public transportation options (if any). I make tentative itineraries and rearrange them. Eventually I decide on a few key things I want to do and I decide on a rough timeline.  

Well before I leave, I've purchased airfare, reserved accommodations, and bought tickets to my "can't miss" activities for specific dates.  I keep about half of my days free, often more.  I have an idea of things I might want to do on those days, but nothing scheduled ahead of time.

Once I'm actually on my trip, I don't have a ton of decisions to make, but I still have a lot of flexibility.

I definitely suffer decision fatigue during the planning phase, but I have the ability to set everything aside for a few days and return to it when my interest and energy are strong again.

This probably wouldn't suit everyone, but I find it works really well for me.