How about teaching undergraduates? I seem to recall from my undergrad days that a lot of my TAs were pursuing PhDs. Is that something that PhD students are expected to do?
Some work as TAs. It is done part-time for funding (tuition waiver, monthly stipend, and insurance). It is not usually a requirement in US schools, although I recommend my students do it for at least a semester to get the experience. Otherwise they are funded as RAs.