I've been a Florida Trail volunteer maintainer for several years. Just about everywhere in the US (can't speak for other countries) there are trails that need maintaining and groups one can get involved with.

For me the draw was that I was looking for some type of volunteer work to do. Trail maintenance suited me. I'm not really good with large groups of people so volunteering in a homeless shelter or teaching prisoners to read might not be a good fit for me, although both are fine opportunities. I enjoy hiking and an excuse to spend yet more time in the woods really appealed to me.