An idea for a rental car that I recently stumbled upon: www.zipcar.com
I live in Brooklyn and therefore really don't have the need for a car on a regular basis but I do need one from time to time or of course for the mandatory get out of the city last minute escape. Zipcar seems to be a wonderful compromise - you pay a (minimal) fee for the year and then can rent a car either for a few hours or for days - whatever you want. You book online to make sure the car is available - they have cars all over the city and the boroughs.. there just happens to be two zipcars in the garage of my building so it is extra convenient.. I'm not sure if the rental fees vary or are the same for every car but you can find that all on their website I'm sure.
Worth looking into for these times when you gotta get away!