Pick something that you are passionate about and/or have been wanting to do for some time and do it. There are options out there for everyone, from joining a community garden to working as a stage hand at your local theater to cleaning up your park to being a loving companion to animals in an animal shelter (and may the list go on and on). The options are endless. I am certain there is something you are bound to love that could not only benefit the growth of your own character but could greatly benefit the growth of your local community and beyond. If you're focus is on your career, find a way to volunteer a little of your time within your field; perhaps you could offer a monthly workshop at your community center or participate in a community event that needs experts in your field. Again, the options are endless. All it requires is a little effort on your part to reach out and make yourself available.
Nothing beats the feeling of helping our community on your own time.