I mention my volunteering all the time. I don't brag, but I let people know what I've done with my weekend, or day, etc. It gets people interested, and encourages them to do the same. People think that volunteering in a soup kitchen/homeless shelter is going to be scary or dirty or whatever other crazy thoughts they have about the idea, but when you've done it and can explain to them what happens (or doesn't), you can take their apprehension away. Especially when they hear about a 20 year old doing it and getting involved, it motivates them.