I’ve traveled to many corners of the planet and have a different take than most. Many people try to min/max their trip, filling up every minute of every day which doesn’t appeal to me at all. I prefer a laid back, impromptu schedule to give myself time to see and do stuff I didn’t plan and time to breathe and enjoy being in a new place. To me, the worst thing you can do is overplan and overschedule so you’re stressed out if something happens to screw up your tight schedule.
As for selecting what to do, I usually do tons of internet and book research finding things that sound interesting. I add everything to a list and to Google Maps as saved points and then try to cluster them into days, making sure I’m not packing in too much as noted above. I’m not especially concerned if I don’t get to everything–if I really enjoyed a place, odds are I’ll return and put focus on different experiences.