Chicago and Philadelphia are great cities for what you're looking for. For sightseeing, I'd pick Philadelphia (more historic places). For food, Chicago. If you do Philly you could even rent a car and go to DC for a day or two. It's around a 2 1/2 hour drive.
Does it have to be an Eastern US city? I just got back from San Francisco a few weeks ago and I've been recommending it to everyone! Loved it there, tons to see and do, and a FANTASTIC area for food (the city, Napa Valley, Monterey). Just not quite as much history as a place like Philadelphia.
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