What I'll say probably just elaborates on themes in posts you've surely read so far about Wash U. We really do have a nice balance between studying and having fun. Also, since finals start really soon here, you do see a lot of people studying in groups (whether the same subject or not). People like to be in each other's company at Wash U; we are a social and collegial bunch, but not in a partypartyparty24/7 kind of way -- although there are some here that do this.
In some ways, Wash U provides an extension of the type of social environment you enjoy now.. only you're older and more independent and have more things to do on a college campus and surrounding area. Because of this, and because of the diverse group of people here, it also means that there are (as mentioned above) opportunities for you to just party a lot or just study a lot -- because there are people in high school that do that now. Still, 90% of people have a nice balance of both extremes.
I'd say my experience here is typified as low-key fun. I don't go to many of the big frat parties, and that's fine for me and my friend group. Having fun on campus can be going to our student group events, a lecture, board games, or movies. Alcohol may or may not be involved in any of those just mentioned activities. Off-campus, I'd say going to the Loop is most popular in terms of neighborhoods, with CWE also so. Forest Park is a lot of fun when the weather is nice because of the free zoo and museums and great paths for jogging. Freshman year: your social life will most likely revolve around your freshman floor community, and I loved that aspect. Upper-classmen years: most people are super involved in various on-campus groups... so you have less time to do "true"(?) social activities, but somehow you still have fun along the way.