One thing I do that is a significant waste of time is something no one can see. It's something I can do while on the bus, while in a job interview, a meeting, or talking to a new acquaintance.

This thing is not paying attention. Not paying attention can comprise of a million things. My most popular are fantasizing, simulating conversations/events/arguments that will never occur, complaining. What are yours?

The time spent not paying attention could be spent paying attention to the external world rather than project what I'd like or fear to happen. It could be spent really listening to others (and being in a better position to help), cultivating curiosity, going over my Spanish vocabulary, ect.