What are some examples of "service activities" for the inexperienced/up-and-coming academic? I am looking for an assistant professor job in engineering. While working on my Ph.D., I had the chance to mentor graduate students in my research group (suggesting experiments, helping them analyze results, etc.), and I also mentored an independent studies student for a summer. Would these types of activities even loosely be considered as "service activities" by a search committee, or, if the question comes up during an interview, just say that I don't have any such activities to mention? If the latter, what are some good ways to respond?
As for the question in the title, some common examples would be organising a seminar, helping to organise a conference or workshop, or serving on a committee.

I don't think that the activities you mention are generally considered 'service'.  They are definitely activities worth mentioning in other contexts, though.

If you really don't have any experience in 'service activities', then perhaps a good way to respond to questions about service would be to admit that you don't have such experience so far, but express interest in gaining such experience (with specific details of the kinds of service you are especially interested in).  I have no idea whether this is good advice, though, as I'm only a postdoc myself and have never been on a search committee.