The big issue is the nighttime stuff, really - I guess I was hoping someone might have a magic alternative to all the dinners out at restaurants?
I'm in my early 20s and most friends are early to mid 20s. My best friend is actually very frugal but is a newlywed who works all hours and she and her husband are able to be frugal by working a lot and mostly keeping their own company. The rest are very spendy. As a young single person it's frustrating when I don't even really like/want all the meals out and bar drinks ($3 for a club soda!) - but would like to maintain current friendships and meet new friends and of course, potential partners!
My wife and I have lived in NYC for more than a decade, and she works in Big Law. I work in finance. So this isn't just theorycrafting - we've pretty much lived what you're talking about here.
1) Meals out are pretty much the most expensive thing you can do in NYC. You won't be able to avoid them all the time, but with your close friends, you can definitely sidestep them. Avoid them by scheduling stuff like dinners at home, movie/takeout nights with friends, parties in your apartment where you mix fancy drinks or serve craft beers/homebrew, etc. If you don't know how to cook, learn to cook!
2) Going out to $125 dinners twice a month with your friends isn't gonna kill you on a Big Law salary. But going out to dinner five times a month with friends, plus takeout for yourself four nights a week, plus your FreshDirect groceries, plus Starbucks every day, plus buying a chop't salad for lunch every day - that set of habits will actually be really, really costly. Extra penalty points if you elect to live in a super expensive apartment in midtown for just you. For us, we make sure that our personal spending decisions that impact only us (our rent/where we live, morning coffee, workday lunches, grocery shopping, dinners at home) are all frugal and low-cost. This way, if our friends want to go out to dinner at a steakhouse or something, it really doesn't phase us.
We're good enough with our "personal spending" that we don't need to curtail our "social spending" in order to save money. This is - by far - the best way to 'stash money in NYC. Nobody but you will notice that you never buy lunches out and you always bring coffee, or that you have a roommate in Brooklyn even though you're 26 and make $160k/year. But everyone will notice that you NEVER go out to eat, even though your close friends/colleagues are all going.
3) Be a good host, as best you can. Make your apartment a cool spot to hang out. Keep some decent booze around (for us, that means good boxed wine + craft beer + small selection of hard alcohol). Learn to mix cocktail drinks, and plate food so that it looks tasty. Get some interesting boardgames, cardgames, and set your TV up so that more than 2 people can watch it. If you can make it so that your apartment is a fun place for a handful of friends to hang out, no one will care that you're not at Nobu. If going to your apartment for "dinner and drinks" feels cheap and lame, suggesting that you hang out at your place won't go over well. If watching the game at your apartment involves free/low-cost food and a setup that is better than the average sportsbar experience, your friends aren't going to blink when you volunteer to host the Superbowl party instead of going to some dump on 2nd avenue that has $12 pitchers and fratboys screaming in your ear.
4) Keep an eye out for the low-priced stuff you can do in NYC, and make it a point to do some spendy stuff when it goes on sale. Restaurant week and broadway week (which just started yesterday) are good examples.