You say you want to be more mentally productive, but may I ask in which particular way? Say, if you sign up as a volunteer literacy tutor and teach someone to read, would it count as mental or do you need something more highly intellectual than that?

If tutoring is up your alley, then your local public library, adult education school, senior center, teen center might have volunteer opportunities posted on their websites.

Note that LinkedIn allows now to add your volunteering to your profile. So volunteering can even benefit your chances of future employment by enriching your LinkedIn profile.