Agree with others that a good part of your weekend will be spent on traveling from Seattle to NYC. Not sure if any airlines have a direct flight to JFK or LaGuardia, but if you do decide to go you may want to fly into Newark, NJ (airport code EWR) on Continental Airlines and take the train into the city. Continental should have a direct flight from Seattle to Newark since Newark is their hub.
www.orbitz.com usually list some good prices for airline tickets and hotels. Use that to check out the prices then go to the airlines site directly to price the same flights. Most US airline sites guarantees to have the cheapest prices online so it wouldn't hurt to compare their prices to orbitz.
NYC is a fun place, but you'll barely scratch the surface in 2 days. If you do go, you can buy a daily pass for unlimited ride on their public bus and subway system for $7. Make sure the pass is for the MTA. Taxi is fine, but it can be costly.