When I was reading reviews as a prospective student, a lot of people were saying that there's nothing to do here, that South Hadley is How Sadley. My experience has been completely the opposite. How much you enjoy yourself here seems to be dependent on what your expectations are and how you personally define social life.
As for me, I find that there are so many things to do any given week on campus. I'm not really a party person, but there are parties on campus every few weeks, and there are constantly parties at other colleges in the area (accessible via the free bus system). MHC is well known for Las Vegas Night in the fall, and Drag Ball in the spring. The latter is soooooo much fun.
There are constantly speakers, student-run events, student organization meetings, and shows on campus. If, for some reason, none of this interests you, there are a world of events in the Five College consortium. The cultural shows and events are particularly exciting. The only one I got to attend was African Caribbean night, but there are also Korea night, Hawaii night, Chican@ week, and VariAsians, for example.
If you're into sports, there are plenty of opportunities to play! We have common sports such as basketball, tennis, and soccer, but we also have rugby, ultimate frisbee, squash, and horseback riding. There's even a group that's trying to make quidditch official. As a Division III college, sports are not the priority of most students. They're just for fun.
Plenty of women's colleges are known for their traditions, and Mount Holyoke is no exception. A common theme with many of them is that they involve reflecting on youth while preparing to move forward in life. Milk and Cookies (M&Cs) is one example. It's essentially snack time. Every school night, there are snacks set out in the dining halls or kitchens. The selections change every night, and they include cupcakes, granola bars, tortilla chips, fresh fruit, and, of course, cookies.
Mountain Day is another good one. It's a surprise holiday. One day in the fall, the clock tower chimes more times than it should. That is the signal that classes are cancelled for the day! There are buses that periodically travel to Mount Holyoke (the mountain, not the school, of course)! You climb the mountain, and when you get to the top, the president of the college is waiting with ice cream!
The Big Sister/Little Sister tradition is my favorite. Members of the junior class and the first year class get paired up to be sisters. This is not a formal or enforced tradition at all. You can decide who your sisters are on your own, or you can ask the junior class representatives to pick one for you. The relationship can be as deep or as shallow as you make it. I chose my own. I technically have two big sisters, but I consider all of my close friends to be family at this point.
All in all, deciding what to do can sometimes be a challenge, and sometimes I have too much work to do anyway. But if there's something in which I'm particularly interested, I'll take a break from my work to enjoy myself!