Not sure what the public opinion is on this, but I used to volunteer with Union Rescue Mission occasionally, and through my church I've had a few chances to help out with other orgs and shelters.
There are groups that do trash pickups, especially around the hiking trails and public spaces.
It was a one-off, but my work had a volunteer service day last year where about 15 of us showed up to help out at Catholic Charities of LA over in Long Beach. We helped hand out food for lunch and pack up food bundles for the holidays.
Not sure about how available for commitment you are, but if you're good with kids and are down to really put yourself out there regularly, look into Big Brother Big Sister. It's one of those things I know can make a big change in a kid's life, but I've never had the commitment or drive to volunteer myself.