Harper
There's all manner of clubs on campus. But unlike a club that you might be used to in highschool you can think of these clubs more like mailing lists, stuff happens, someone broadcasts it, and you can join in if wanted.
Organizations which are more comparable to highschool clubs are Olin's student government, Mini Baja build team, Human Powered vehicles build team, and the Honor board.
Olin has only one large proto-frat/party throwing organization. Manhall. Located in a hallway in east hall and started as tongue in cheek reference to traditional frats it has become Olin's sole provider of "large college parties". The parties are well run, safe, packed full of people, and the guys in manhall make sure everyone stays safe and isn't creeped on. Without Manhall Oliners would be likely meander over to Babson's roofie infested frat parties full of over boozed up bros or drinking in small groups in someone or another's dorm room. A big plus for people interested in women, Manhall parties are full of Wellesley women who are there to have a "good time" and who are there in a 5 to 1 female to male ratio.
Dating at Olin (olincest) is fairly common. As the gender ratio is so high your likely to bump into someone who's fairly attractive and worth dating. In fact a large percent of Oliners date within Olin as the population is so accessible and interesting. (And surprisingly not terribly full of neckbeardy kind of people) If you're not dating an Oliner you're likely to be dating a Wellesley student. There are 150+ of them at Olin every other weekend at the Manhall parties so you'll have your fair chance to meet them.
Casey
I've been working in the admissions office this summer, and the question of clubs and activities comes up often. I think prospective students are frightened that because of the small size, there will not be a club or group geared towards what they are interested in. But since Olin students have a very diverse set of interests, that is definitely not the case. There's a rumor that at one point there were more clubs than students! Some of the more popular ones include the Olin Conductorless Orchestra, Mini Baja (design, build, and race a little car), Human Powered Vehicle (the same as Baja but with a bike), PowerChords and Ol'Enharmonics (both a capella groups), Olin Fire Arts Club, Franklin W. Olin Players, a soccer team, an ultimate frisbee team, and SERV (a community service group). Some of the more interesting clubs include Cheese club, the Olin Rock Orchestra, All Your Bass (an a capella group focusing on memes), Uni (the unicycle club), and a recently started kazoo marching band.
Lots of kids cook or bake, and there's a ton of dancing, whether it be swing, tango, or late-night dubstep in a lounge.