Lunch in/near Princeton
Since you say that "a bit north would even be better" I'm going to suggest Sophie's Bistro, which is in Somerset, just before you get to New Brunswick.
I am a big fan of Sophie's and I think it would meet your requirements. Do not be put off by the restaurant's location, which is in a light industrial part of town. The food is wonderful as is the ambience. FYI, here's a brief Chowhound discussion of Sophie's (there are others if you do a search of the board) along with their web site: