Depends on the environment at work (do you have privacy) and your interests.
- ebooks on your phone or a reader (or even a physical paperback!)
- games on your phone, or a small console like the retroid pocket 5
- learn something (new language, watch a lecture course, etc)
- learn + practice some IT skill (programming, website design, etc)
- learn + practice any other acceptable skill (drawing, writing, etc)
- watch movies/TV shows
- get home tasks done to save time at home (home admin, pay bills online, etc).
- chat with someone you know