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The Oyster Bar is a great choice, and prices are on par with other Manhattan seafood restaurants. The other upscale restaurant in Grand Central is Michael Jordan's Steakhouse, but I can't offer a recommendation as I haven't tried it yet. For drinks try The Campbell Apartment, named after the family who lived in Grand Central Terminal (note: the bar has a business casual dress code). For more casual fare the dining concourse on the lower level has something for everyone, including Junior's (they have a take-out bakery stall on the upper level too). For foodies with a gourmet appetite, don't overlook the Grand Central Market, offering fine cheeses, meats, and other prepared foods to take out.
On a side note the New York Transit Museum maintains an annex at the station, located adjacent to the Stationmaster's Office (Shuttle Passage side of the terminal). Admission is free, and they have rotating displays culminating with the holiday train layout (from November to mid-January; the layout was featured in previous issues of Classic Toy Trains).
Although not mentioned in earlier posts, the PBS special "Secrets of Grand Central Terminal" is worth watching before your visit so that you can fully appreciate the grandness of Grand Central Terminal. NYC Architectural Historian Barry Lewis is another great resource, as he has lectured extensively on Grand Central Terminal and its fallen sister, New York Pennsylvania Station.
-John