I don't think it has to be unique. Lots of people do the same things over and over again.
I did a lot, I volunteered with LGBT causes, cleaned beaches, volunteered about 3000 hours for a space education non-profit, tutored, did light repair work for the ill (basically painted houses), volunteered in a hospital (of course), did historical reenactment (4 leadership positions), and did community service through religious groups. I know there were a few others in there but I had community service/volunteer work drilled into me as a child so I kept with it as an adult. The hospital volunteering, the space education, and the historical reenactment were on my AMCAS. With the exception of two of them, I did very willingly and kept with it for a long time. I could do it because they were great causes/passions of mine.
I think it would probably be best if you picked something you enjoyed and worried less about standing out. Not everyone can be a leader in community service. Do you love helping kids and fancy yourself a decent teacher? Maybe tutoring inner city kids is good. Love museums? Volunteer there. Like renaissance festivals? Contact your local one and see if they sign off on community service as a NPO, I know at least one that does and another organization that does as well.
