If you exhaust the Time Schedule, or are looking for something that doesn't adhere to the start of an academic quarter, check out the Stanford Directory. The directory for the current year lags the beginning of the year by at least a few weeks (if not a couple months), so you might need to look at a copy from the previous year. There are copies in the Greenberg Room, Kyle's office, Gina's office, Michelle's office, professors' offices, and most of the student offices and residences; in short, they're ubiquitous.
Also, check out the listings of Voluntary Student Organizations. There are many. Although most campus organizations are organized by and aimed at undergraduates, none exclude graduate students (with the possible exception of fraternities and sororities, although for years Phi Sig, an on-campus house, included a number of graduate students). Some are aimed specifically at graduates, e.g., the Graduate Women's Network. If you find that the contact number is out-of-date, you can contact the Office of Student Activities (723-3542).
Community centers on campus, listed under Community Awareness in the Stanford directory, serve the interests of special interest groups on campus and are focal points of activities for those communities: the Native American Cultural Center (first floor of Old Union), the Asian American Activities Center (A3C, second floor of Old Union), the Black Community Services Center (Black House), El Centro Chicano (the Nitery in the Old Union), the Women's Center (first floor of the Fire House), the Disability Resource Center (first floor of Meyer Library), and the Lesbian Gay and Bisexual Community Center (LGBCC, second floor of the Fire House). There is also the Out in Linguistics mailing list that you might want to subscribe to: Write to outil-request@csli.
There are also a number of opportunities for public service. Check the directory under Public Service or find the Haas Center for Public Service (currently being expanded, but probably still open at Owen House).
If all the written directories and contact numbers fail you, talk to other graduate students to find out what keeps them busy ``after hours.'' You might be surprised to find out that there are more tried and true ways to distract you from writing your QPs and thesis than you thought.
In sum, Dorothy figured that she didn't need to look beyond her own backyard to find her heart's desire (Wizard of Oz, 1936). You might not find your heart's desire, but you can certainly find a lot of activities to enrich your graduate program on this big lot they call ``The Farm.''