The United Religions Initiative (http://www.uri.org/) is specifically about grassroots involvement. It is organized into "cooperation circles" which work on whatever project they are interested in. A CC has to have at least seven people of at least three different religions. I don't really have any specific suggestions for CC's but there might be one that would suit.

In a general activist vein, I've been considering doing the regular Wednesday vigil that the San Jose Peace Center (http://www.sanjosepeace.org/) holds in Cesar Chavez park downtown. Wanna check it out with me sometime in January?