Working on designing or inventing instruments that measured things that have never been measured before. No one I would call a mentor, but there are certainly people I admire and like to interact with, whose comments on my papers I really value. I had a series of bosses that were willing to give me responsibility and take a chance on me. It takes a lot of work - work hard. Going back to when I've seen young people who work for me, I often have to tell them to slow down and do it right.
Quality is more important than quantity.