I don't like going out to the streets to do things, because that just has a bad reputation, and don't think it has too much of a success rate. I tried leafletting once around my college campus... I just felt like a weirdo, walking around handing out pamphlets to strangers, and I didn't get the thrill of making a progressive effort. Also, for those things, it really helps to be charismatic, and I'm much more introverted. I think a personal connection is important in persuading someone, and it takes time for me to establish that, ya know?
I do, however, insert vegetarianism where I can in personal conversation, and through academic outlets.
I like to write essays on the topic, when I get a broad enough subject to encompass it. I had a sociology professor once who, after I turned in an assignment on world hunger, read my response to the class because I made a solid point about the inefficiency and wastefulness of growing meat for food. I also recently gave a speech on vegetarianism and ecological sustainability, and it seemed to actually reach a few people in my class.
I think you could still call it activism, though. I feel a lot of our ideas of activism are centered around the "extroverted" approaches - the protests, leafletting, public exhibitions - while the "introverted" efforts kind of get overlooked - gathering and synthesizing information (through writing, namely), and small-scale conversation.