I second CitiBikes for getting around/shorter distances (They do have a daily pass, but it's kind of pricey). For longer rides or more frequent use, investing in a beater bike may be more cost-effective- plenty of used bikes are available for reasonable prices in NYC, especially Brooklyn. $50 is doable if you look and can get you something functional; It's also cheap enough that you don't have to worry about re-selling it when you leave - only a little more than a CitiBike day pass.