Being a part-time New Yorker, I can help!
It's so massive, it's almost easier to tell you what not to do, but my first rules are walk and walk and walk everywhere. A plan is good for a some time, but just wandering will lead you to discover things you never knew you'd want to see. Even knowing better, I head into town thinking I'm going to do this that and the other thing, and I end up maybe sometimes doing one thing. So pick a highlight of the day and let the rest unfold.
I'm no snob about the touristy things either. Visitors get to see and do things people who live in Manhattan never do. I have friends over from the UK often, so I'm used to being tour guide and also being a bit jealous of some of the things they've done.
And don't be afraid of the subways. They're not pretty and they can be confusing, having started as independent, competing lines and being run into the ground lately. But it's a simple fare system (use a card, pay upon entrance) and at many stations, you can simply turn around or change lines if you mess up. Ask for the free map at the booth (but don't hold maps on the streets).
Cabs are worth it if it's late, your tired and there's 3 people, cause subway would be $6+ anyway. It can be like an amusement ride - very frightening, but you'll want stories to tell.
There's all kinds of niche website with various literary walking tours, music guides, that sort of things. Sadly, my old record crawl has just about disappeared. But if you do get to do a little vinyl hunting, you can still find cool stuff.
As far as hotels, there's a few I stay out when I can't book a friend's couch. Pretty much, they're listed on the Frommer's guide under the budget hotels (Larchmont, Hotel 17, a few more). You can also score deals online, but I strongly suggest, if you see something on say Priceline or one of those, you go to the hotel's website and check it out directly there and on guide sites to make sure of where it is, reputation, etc. I've had friends book 50 miles out of town, or into bed-bug ridden hovels. Just do a little web search. Cheap prices doesn't mean crap and high price doesn't mean glamor. My rule in NYC is that it's so busy and massive, bad restaurants and hotels know there's another fool along soon, so crap survives and gets big money for rottenness.
Food and drink, you can pay a fortune, it is New York, but you can eat amazingly, shockingly cheap. There's just endless places of every type, at every price point, with the best stuff often being the cheapest (pizza, bagels, Asian, etc.)
I'll have a think about what sites to send you to, but if you have any questions, just ask!
Always dress in layers, with some very light summer clothes for September. It can be hazy, hot and humid like the worst August days, but it also can be quite cool. You can't go wrong with jeans, light capris for the ladies, light T-shirts, cardigans for night and over-chilled stores, good rainwear, etc. Comfortable shoes are a must, forget the heels unless you have a limo. Even just going in for a night, I dress in layers, as stores are freezing, but rock clubs are broiling and it always seems to rain more in NYC (but that's just because you walk everywhere, instead of driving).
It leans more towards indie rock and doesn't include everything folkie and older, but Oh My Rockness send out a nice email listing of upcoming concerts. From there you can see the various clubs and check out what they have. There's lots of free shows and in-stores. And if you have the dates of your trip already, you can sign up for TV studio audiences, such as Letterman, although it can be a bit time-consuming, they herd you like cattle for an afternoon.
Instead of big meals, eat when you see cool things and get coffee at all the nice cafes and use the rest rooms there and in big bookstores.
Bring money for new clothes, I think the economy is still in your favor and some places have coupon deals. Our sizes run big, and are numbered about 2 down from the UK for ladies, shoes are a few up.
My list: Central Park stroll, Empire State Bldg., walk all over West Village, East Village, SOHO, Lower East Side rock club crawl, walk over the Brooklyn Bridge (ice cream and chocolate there to fuel your trip), bagels, . Hipster Brooklyn gets silly, but yeah, you can stroll Williamsburg and look for Alex Turner
Oh I could go on.
Do bring patience and a sense of humor though. New Yorkers are nicer than everyone says, BUT it is a city and it has its trying moments, to be sure. Crime is way down mostly where you'll be, but there's nasty people and very sad homeless situation, etc. so be wise. People go about their business. I hold doors open up in the country but in the city people just WALK FAST.
Rock shows can be early or later or in the middle - check with the venue. Tickets to small clubs can be cheap or even free, some caberets have drink minimums, but a night seeing bands or theater can be dirt cheap if you let it be.
Also, you will need ID for bars and clubs, no matter how ancient you look. Try to be flattered.
~ Karen
http://www.frommers.com/destinations/newyorkcity/
http://nymag.com/restaurants/
http://slice.seriouseats.com/
http://www.villagevoice.com/
http://newyork.timeout.com/
http://www.ohmyrockness.com/