Personally I would try to volunteer in things that actually have meaning for you, or at least are interesting in some way.
For example, I love nature, earth, the environment, etc, so I volunteered planting trees and doing beach clean-ups, etc, did some stuff in support of a few diseases that have affected family members. This way not only will you get into it more and feel even better doing it, but if it comes up you'll be incredibly genuine and more passionate talking about it.