On a related note, I once heard someone criticize American life as being overly held hostage by a "live to work" mindset. It's as if any free moment Americans have, we feel obligated to fill it with a "have to do." (And that then sounds like "I need a vacation from my vacation.")
So it was suggested then that when creating any kind of schedule to first plan the "leisure time." What do you intend to do this day for fun? Schedule that and write that out first.
That could serve as the discretionary time you requested.
It's also been noted that sometimes, great ideas come when you're specifically removed from a project. Some have said they write an idea before going to bed, then saying "Let me sleep on it." Others say after hours of concentrated focus, they actually get great insight by either taking a walk to the restroom, around the block, etc. This kind of discretionary time can be scheduled too, if you allocate it first.
I seem to remember one episode of The Flinstones where Fred said to Barney who was serving as a Little League coach, "The first thing they need isn't the Fundamentals, the first thing they need is the Fun!"