When I visited Stanford, Brown, UPenn, UCLA, Berkeley, USC, Oxy, and a ton more schools, the central theme at each and every campus regarding volunteering or giving your time to something other than schools was the following: be passionate about how your spend your time.
During the summer or even a weekend a month, find some organizations that you are truly passionate about. Do you like working with animals? Volunteer at Paws Animal Rescue. Do you enjoy helping people? Volunteer at the Salvation Army! Do you like the environment? Volunteer at Quinta Mazatlan!
Check out this good website to get your volunteering started! Volunteer South Texas!