Things that worked as an undergraduate included checking out a few key community boards (the 2 class buildings I used the most, the cafeteria, the library and my dorm lounge; loads of film showings, music 'coffeehouses,' trips to surrounding attractions and that sort of thing), the school paper (my first school actually had a daily newsletter that had loads of spontaneous sort of stuff, I would think Cornell would have something similar), the school radio station (always an...interesting...brand of people).

There should also be an international student group; mine had a specific international orientation at the beginning of the year and was really fun and low-key and the source of most of my social activities and friends I wasn't in classes or dorms with. I wasn't really a sci-fi geek in school, but my last one had a Society for Creative Anachronism (SCA), also a rather interesting bunch, and they would do these reading marathons of Tolkien books and show Harry Potter movies and that kind of stuff (even though they were really more about the history).

There's also your public library; I'm sure they could point you to interesting things in Ithaca in general. And on that note, this seems to have gone on longer than I'd thought...good luck!