I’ll go first. I always leave one full day completely unplanned. No reservations, no itinerary. Just pick a direction and figure it out. The best stories from every trip I’ve taken came from those days.
It started as having it for a backup day for weather when doing a hike and turned into a thing I do in most of the trips now. I get to venture in places I would never get to see otherwise or do stuff which is not on a regular travel plan.
What is yours?