I'm going on an overnight trip to visit New York City, and am not sure what to do at night (weeknight). Can anyone offer suggestions?

Why not go see a broadway play? here is a website with information about buying tickets. here is a website on things to do in NYC.

A list of just a couple things include The Metropolitan Museum of Art, The Guggenheim Museum, Rockefeller Center including the ice rink, NBC Studio Tour, Radio City Music HallRadio City Music Hall, New York Stock Exchange, South Street Seaport, St. Patrick's Cathedral, Times Square, the World Trade Center Site, a food tasting tour of Greenwich Village, The Empire State Building's observation deck, Grand Central Station, and The Statue of Liberty, Take the ferry to Liberty Island, where she stands prominently in New York Harbor. From Liberty Island go to Ellis Island where our nation's immigrants, eager for a new life of freedom, were processed to become new American Citizens.

Hope that helps. If you want any more idea's just ask!

yafa or someone: tagging on to this topic, I recall when I was in New York there was a quite astonishing array of services and places that operated 24 hours . One which amiused me was a 24 hour bed service i.e. you could buy a bed and have it delivered 24 hours. Ideal for insomniacs who blamed the bed! Not much use to the tourist, I grant, but there are surely other things which are open, not just bars, which are of interest?

Welcome to the pool Sara -
Try these:
http://www.nycvisit.com/home/index.cfm
http://www.nyctourism.com/default.asp
http://www.nyc.gov/html/culture/html/tourist.html
http://www.allny.com/tours.html
http://www.digitalcity.com/newyork/

Yafa, FredPuli, and Methos5000 -
Thank you for all the help and urls! I'm sure this is going to be an exciting trip with all the things I've read about.