I use that time to think about the big plans I'm working on. Not talkin' bout like "what I'ma do next week" but more like "what do I need to do now to attain something 3-5 years from now." Always long term strategy shyt that I can't think about properly during the workday or while I'm chillin' with the people.
Good example is going back for a doctorate. I been thinking hard about the path I need to take once I'm back in the classroom. Two years ago today I was thinking about my strategy for studying for the entrance exam. Today I'm ready for the exam and thinking obsessively about specialties in the field and what the job market projections look like.
Also find myself thinking about if/when I'm going to lock my chick down for the haul.
When my mind is too tired, I just read a book. Something fictional and future based tends to get my imagination sparked on some wild space-travel and human engineering shyt. Puts me right the fukk out.