Hit thrifts and flea markets and pick up some art, usually really cheap, but pick a theme like all landscapes, to add a sense of space to that wall. Usually they even come with frames. If you pick a landscape theme they will all have a common color. Same goes for ocean scenes. They will visually add movement to your room. If they aren't too heavy you can hang them with poster pins that don't leave big holes in the wall.

Another inexpensive idea is to buy a calendar with pictures you like and frame them.

Do you own a camera? New York is filled with interesting architecture, you could take pics in black and white of the neat things in your neighborhood and frame all in black frames. Check $1 stores and thrift stores for inexpensive frames. You can make the matts yourself with poster board.

http://houzz.com/ideabooks/809
This ideabook may help. There are several others if you search ideabooks for "art". I think you could use a rug - try ikea or a home goods/Marshall's type store for inexpensive options.

Find some cool fabric and stretch it over frames or pit in some embroidery hoops painted black for some color.