I'm devoting this post to discussing the different sorts of activities you can participate in on campus. First I must say, there are more student groups on campus than you could ever have time for. I suppose this is bad in a way, because you wont be able to do anything. But the cool part is, if there is something you want to do, there's probably a group that also likes to do that. And if there isn't, you can start one! The main organization to know about is the ASA, the Association of Student Activities. They are the official organization for MIT student groups. You (incoming freshmen) and any other new students (grad students) will receive/have received a CD-ROM and booklet describing all the ASA-recognized groups on campus. Plus, during orientation, there will be an Activities Midway, where you can explore and sign up for different groups. (and I'm pretty sure there aren't any conflicts with that time, so you'll be able to go!) Of course, feel free to contact any group that interests you prior to coming to campus, if you want to know more about the group.
I've had questions about a few groups in particular. Elina asked me about a cappella groups, and Laila asked me about dance groups. In terms of a cappella groups, there are quite a few of them on campus, but each group has their own style/personality, though most groups sing pop music. And besides all of the a cappella groups on campus, there are plenty of other music groups. There are choirs that you can join (and get credit for in some cases!) and bands and orchestras as well. There are also quite a few groups that put on musical productions and plays.
As for dancing, besides performing in musical theatre groups, there are plenty of dance groups. Some groups do hip-hop, swing, ballroom, ballet, tap, jazz, as well as different cultural groups. I was going to list and link to all the groups, but I realized that there are way too many to count. My suggestion is to go to the ASA Lists page and search either by activity type, or just bring up the whole page and look through them. Then you'll realize why I opted out of listing them all here (and decided to be lazy).
This post is definitely not meant to be complete for all the activities, so I encourage you to check out the information on the ASA website or on the CD-ROM they send out over the summer. And if anyone is reading this, and see that I have said something wrong, or left something out, please let me know, and I will edit this post.
Of course, there are plenty of activities that you can be a part of that are not part of MIT. My freshman year roommate participated at a ballet company in the Boston area, and has continued with it these past few years. If you are passionate about something, or if you want to try something new, there are plenty of chances to do what you want during your years at MIT. Whether you go off campus to do it, or just get together with people not as an ASA recognized group, or join an ASA group, it's definitely important to do stuff outside of school (there I go again, lecturing you like I'm your mother or something). But it's true.
Okay, I have other stuff I want to talk about, but I'll leave that to another post, since it's getting late (because I have to wake up so early), plus that way this post is just about activities.
-Stephanie