Somebody who learns a new skill and thanks you for it?
Or - I know I thank my boss when I get to work on the things that I like, or when a complicated interaction with people outside the team is handled well.
Honestly it's really quite satisfying, just to unblock somebody. At the IC level usually it's small things like figuring out why the dev environment won't load or the tests keep failing, but they're thankfully and it's fulfilling to me.