For the past month or so, I've been living about a fifty minute commute away from the place I work. It's not ideal, but it means I have almost two hours each day to think while walking or on the bus.
Some thoughts aren't worth much, like when I think about how long it'll take me to get home. I'll reassure myself that I'm five minutes closer than I was five minutes ago, and then try to remember how I felt five minutes ago when I had so much further to go. I'll try and guess what time it is and then look at my phone to see if I'm right.
Sometimes I hum a tune, the sort of thing I used to put down on paper or on my computer as soon as I got home. I still try and work out what the notes would be if were to write it out.
The rest is hard to describe. I think about things I need to do, planning out emails to send and schedules to follow. If I'm hungry, I think about what I'll eat. Sometimes I think about ideas I disagree with and try to identify why I disagree. It's easy, of course, to win arguments in your head. I forget a lot of what happens on my commutes, but if I arrive and time has passed quickly, I know I've had a good think.