Some possible synergistic activities, which are within reach of many people:

Mentoring members of minority or underrepresented groups. (At any level: undergrad --- e.g. an independent study or senior thesis student, grad, or post-doc.)

Giving talks/lecture series/mini-courses at events aimed at undergrads or grad students.

Organizing events aimed at undergrad or grad students.

Creating/maintaining wiki's on various topics.

Helping train your institution's Putnam team.

I think of all of these could count as broader impact as well.

For a post-doc or junior faculty member, advising students in independent study, and/or advising senior theses, are among one of the easiest (in the sense of being available as an option) broader impact/synergistic activities that one can undertake.

I think that online participation can count as broader impact, but it should probably be substantial enough to be noteworthy. For example, maintaining a prominent blog would certainly count (I would think both as a synergistic activity and as broader impact).

Organizing a workshop/conference/etc. could count as broader impact. I've known instances of fairly young people organizing (very successful!) workshops (e.g. at AIM). More commonly, as a junior person, you could be part of the organizing committee along with more senior people. There are conferences being organized all the times, at AIM, MSRI, Banff, etc., so there are definitely opportunities available here.