Among the most beneficial ways to volunteer is to mentor a bright boy.
Endless programs exist to mentor and otherwise help girls and women, far fewer for boys, even though boys' school and life performance, relative to girls, has so declined in recent decades. For example, in 1980, the ratio of college graduates was 50/50. Now it's approaching 40% male, 60% female.
I advocate helping bright boys rather than boys in general because the public schools are now so focused on helping the lowest achievers that bright kids' great potential is so often being wasted. They're forced to sit stultified and if they can't, they're too often put on a Ritalin leash.
How you mentor can vary. For example, you could do a recreational activity together: build a model, play basketball with him, whatever. In the process, bring up topics that might help him identify interests, reveal problems, come up with questions to ask you. You could meet once a week or once a month, or not meet at all--mentorship by phone and email counts.
To find a bright boy who could use a mentor, contact the counselor at a local school, the gifted child coordinator in your school district, the principal of your church or synagogue's Sunday School, or simply ask your friends or PTA members.