What are the most popular student activities/groups?
Ah, well, I can't speak to what's most popular, as I have not collected data and was never a popular guy, haha. But I think what's more important, anyhow, is how many different groups and activities there are. I know that in order to be an official student organization, it must meet certain criteria, so one is almost guaranteed to not be wasting their time if they're following something they have genuine interest in.
I admittedly met most of by close friends through the music department, so that somewhat affects what I do on and off campus. For example, on weekends I usually play a gig with a soul/funk band at a local restaurant/bar or music venue, or rehearse for one. As for being around alcohol at a gig, I would argue that that's a safer way to be in that environment, since the bar, being a business, does not let anyone under 21, myself included, drink. At all. Outside of that scene, I've dabbled in ballroom/swing dance club. And there's always a good show to catch on most any day of the week, whether it be music, theater, or other form of art. Best of all, these are usually discounted for students, or even free with VCU ID!