There is a group/activity/club at Grinnell for everyone. Tons of people are involved in intramural sports or social activist groups. The most popular (and most respected) sports are basketball, track, cross country, swimming, and soccer. Frisbee is insanely popular. But, I wouldn't say athletic events are all that popular over all. Lots of people graduate without having ever seen a single game.
I'm involved in theater, and the great thing about it is that you don't have to be a major to be cast in Grinnell productions because a lot of them are student-run. Also, theater attracts all sorts of people--athletes, singers, political activists--you really aren't defined by one activity.
The dorms are very close-knit communities. Student almost always leave their doors open, and floor/dorm parties are frequent and fun! People often study in the hallway, and it's rare to find a person who spends a ton of time in their dorm room. Because Grinnell is a wet campus, it's common to see alcohol in the dorms. On a typical Friday night, you'll probably see a beer pong game underway in a dorm hallway, and a party in a dorm lounge.
There isn't much of a dating scene at Grinnell. People don't really "go on dates." It seems like people are either casually hooking up or in a committed relationship. But there are lots of opportunites to meet people.
Grinnell is really unique because is tends to party as a community. For example, there are several parties (10/10, Waltz) that are progressive in which different dorms host different parts of the party. On nights like 10/10 (the weekend after the first campus job paycheck), you can literally see almost the entire campus travel from one dorm to another. Each weekend there's a college sponsored party in a big multi-purpose building called Harris. There's free beer and lots of dancing. The party has a different theme each time, and the most popular ones are Disco, Fetish (dress up as a fetish), and Halloween. Basically these parties are an excuse to show some skin and drink.
Students at Grinnell don't go off-campus much because there's so much to do on campus--there's a free movie theater, parties, guest speakers, free athletic events, free theater and music performances, free concerts--really anything you'd want. There aren't many bars in town, so most people drink on campus. There's even a pub on campus.
I met my closest friends in my dorm freshman year and in my classes. The dorms aren't separated by year, so a lot of my best friends aren't the same age as me.
If you're awake at 2am on a Tuesday, you are most definitely studying.
There are no fraternities or sororities at Grinnell. In fact, that whole college lifestyle is pretty looked down upon, and there isn't really even anything at Grinnell that compares to it.
Drinking is really common, but there is lots to do that doesn't involve drinking. People will often cook in the dorm kitchens, watch a movie, go to a concert, or just hang out. Actually, lots of people who don't drink still go to parties and have a good time because you aren't really looked down upon if you don't drink.