It’s a bit of both - I absolutely find it fun! I think the retention rate above says a lot as well to that, but I’ll try to give a picture of my typical week and hit your questions on the way 
When it comes to sports, that’s not me or most of the school, but there’s probably about a quarter of the school that’s more into that, usually via Hockey/Basketball which are the two main sports here. Generally, though, the school spirit is expressed in a shared mindset of practicality more than through your typical school spirit. I think people often conflate practicality and pre-professional when it comes to Northeastern - I’m a very practical person but I hate the corporate world frankly. While there are certainly more pre-professional people here too, there are plenty here that are interested in academic subjects for the sake of it and usually minor or double major accordingly.
I have a philosophy/ethics minor and plan to eventually get a Ph.D. after I retire, just for fun. But to make money to enable that, I’ll work in my career in CS, which I also enjoy.
The STEM programs here do tend to be bigger/better funded, but there are still plenty of good non-STEM programs. The philosophy department at Northeastern is actually pretty amazing in my experience, of course specializing in applied philosophy, specifically ethics.
In terms of typical days on the quad, that’s absolutely a thing when the weather gets nice. There’s also plenty of igloos, snowmen, and some other fun snow shapes in the winter. The difference with Northeastern is that student life takes advantage of the full city. On sunny days, you may find hundreds on the quad, but you’ll also find them in the Boston Commons, kayaking on the Charles, running along the Esplanade, walking Newbury, at the SOWA market, etc. As many things happen on campus as off, so usually people do outings with friends all over the city. As a fourth year, a few friends and I have a regular brunch setup where we go and try new places (usually still college level cheap) for brunch weekly or bi-weekly. Even if it’s not a perfect day, people will go find a cool coffee shop and hull up or do tons of other activities. I personally rock climb (indoor gym) and try to make it out a few times a week with friends. There’s a huge Facebook group you can post in to see who’s going to be at the gym when you are, and often people arrange weekend trips for outdoor climbing via that too. My best friend and I will regularly go out and just explore a new neighborhood just for fun.
Everyone lives on campus the first two years, and you don’t go on co-op until your fourth semester or later, so your first year would be indistinguishable from any other school. Dining halls, dorms, clubs, etc. There are a few small differences that make co-op easier though. Clubs work by semester instead of year, things like that. While people go elsewhere for co-op sometimes, a good deal of people stay in Boston and are still there when it comes to social things, and people tend to wait until 2nd/3rd co-op before going elsewhere. Even when you do go elsewhere, most major cities have concentrations of Northeastern students, and our co-op connections office sets up events in those cities. There’s usually Facebook groups too and people go do trip in the area. I was in LA for a co-op, a good friend of mine in SF, and I flew up a few times and hung out with Northeastern students in SF. Some of them are now my better friends in Boston now. There’s definitely some coming and going from campus, but everyone’s in the same boat and the social life adapts. I have a friend I’m hanging out with tomorrow that I didn’t see for a year between co-ops outside of Boston, but we’re still very close just as when we met freshman year.
As far as planning and pre-professional go, I like to say that every Northeastern student has a plan, but it’s always subject to change. People go on co-op and change majors or career direction or maybe they love it and get that confirmation years before graduating. The point is that you don’t have to have it all figured out, but people here all tend to have an eye towards it. Not because they’re all money hungry and anxious to put on a suit and work 9 to 5, but because they want to be happy when with their career when they do actually have to do that. Or maybe they realize they can’t do the 9-5 and explore starting their own business, being a freelancer, etc. But you get that 9-5 work experience with co-op so you can decide for yourself what works for you.
Hopefully that all gives you an idea of what it’s like