A classic of Grand Central Station, but so often neglected by us New Yorkers, is The Oyster Bar. The two times I've ever dined there (using the term "dined" somewhat loosely when you're sitting at a counter bar for a total of 45 minutes while waiting for a train and a bowl of chowder, hoping the latter arrives before the former) were with friends who live in Washington, DC and in Paris, at their requests. For some reason (perhaps the middling quality of the cuisine?), this isn't a place that quickly comes to the minds of those of us who live here.
OK, is the food the greatest? Nah, but the seriously classic throwback-to-another-time ambience and those Guastavino ceiling tile vaults make it a must-do at least once in a New York visit, even if that visit has an open-ended return to somewhere else (ie., you live here). And, if you're into seafood like oysters, clams, mussels, or even for that quick bowl of chower, it sure beats the noisy food hall in Grand Central's lower level - for recognition points back home, relative comfort (there's plenty of seating) and speed of service.
At least once, and take pictures!