It is always a good idea to have some form of official ID (passport, drivers license) with you at all times in New York, especially if you are driving, are doing big personal financial transactions, or want to go to a bar. (Unlike most other places in the world, the legal drinking age in the US is 21 years old - and it’s enforced. Bars and restaurants in NYC take the law very seriously.)
As carrying your passport around with you all the time runs the risk of loss, a driver’s license with your photo and birth date on it is best, but unfortunately doormen, police officers and the like might not recognize it if it is from a foreign country.
If you can, we’d suggest getting a New York State non-driver photo ID card. It looks just like a New York drivers license and is a great form of ID, costing around $10-15, depending on the term of use. However, it’s a real a pain to get together all the ID required to add up to enough ‘points’ (read carefully here) and you will have to queue at the DMV.
For more details on this, check out this link http://www.nydmv.state.ny.us/license.htm#nondriver.