As a guilty party who often tells her guests not to bring anything (and yes, I really do not mind if they don't. Almost everybody does not). Here are some suggestions for when you just can't bring yourself to show up empty handed.
Wine - Always welcome, always useful. Most wine stores in the neighborhood have a small kosher section. The one near the Hunter College Subway Stop usually also has decent prices. You do however run the risk that several other guests will also show up with wine. Do not be insulted if your particular bottle is not opened. There's only so much people can drink. If your hosts are not big wine drinkers I recommend Moscato - fondly called the blue bottle. ALmost everyone like it and if they don't, they can tolerate it.
Flowers - everyone likes getting flowers and they perk up the table. Just make sure to bring them over before shabbat. This is not a great idea if your host has strong allergies. This is also pricey and may seem extravagant as most flower shops around here are not cheap.
Challah - If you are actually told to bring something, this is often the requested item. In days of yore you could go to Gristede's and get a nice Zommicks, but alas they have switched brands. Don't even think about food emporium. They have Challah but it isn't kosher. You could buy one of the enormous ones from bagels and co. but not everybody loves the way they taste and they really are massive. Park east will also have challah. If neither of these two does it for you you may have to head cross or downtown to find a better option.
Dessert - Another common request. I hope you have an oven because there is not much leeway around here when it comes to purchasing pareve pastry. bagels and co has a small cookie selection which is not great for this purpose and will cost you an arm and a leg. Supply at Park East varies from week. Your best bet is to make something. Plus it makes you look good. The ultimate fast dessert? Apple crumble. The slowest step is peeling the apples and everyone seems to like it. You can throw in cranberries too if you're feeling fancy. fresh fruit is also really nice if you have the time to cut and arrange it. Skip the chain store and buy from the cart guys. All carts are not created equal so you need to get familiar with the vendors in your area. My preferred vendor is on 70th and york. If you know of when with good products, let us know!