I've tried some volunteering and haven't had a ton of success finding good volunteer opportunities. It seems like the food bank has some options, but they're way out in the zona norte.
I spent a weekend volunteering with Techo building a little kit house for a guy who had been living in a scrap metal shack up in Pilar. Mostly I felt like I was way older than the other volunteers and I wished they'd have let me go home each evening to sleep in my own bed instead of on the floor of a school, but the work seemed to be useful to the guy receiving the house: https://argentina.techo.org/voluntariado/
I have previously volunteered with Fundacion Sí who organize recorridas nocturnas to chat and feed those living in precarious situations here in the city. They have new volunteer meetings frequently. There are many organizations that do those recorridas based on barrio and organize via Facebook (recorridas nocturnal Colegiales y Chacarita is one of the groups).
As Julia stated above, there are food banks in many barrios who are serving those in need, you can sign up for cooking/serving shifts though some have closed as they relied on state funds that have been cut. Those are the opportunities I'm familiar with. There are also organizations that organize a sort of boys/girls club events where kids get mentors to hang out with. But the one organization I knew of doesn't seem to operate currently. (Club Union de los Pibes).
I just read how the valedictorian of my alma mater (Notre Dame) is returning home to Buenos Aires to work at this foundation: https://www.lanazarena.org.ar/
It's outside the city a bit, in Florencio Varela. Their website indicates they encourage volunteers.