Serena C.
Yelp
I'm surprised that no one's mentioned that Nassau Street Seafood makes the most amazing grilled fish sandwiches in the back. Yes, it's a bit strange that a fish place has a grill and cooks the fish they sell, but it really does make perfect sense--particularly once you eat it.
I recommend the grilled tuna sandwich (I prefer it rare or seared, but as they say, eat undercooked seafood at your own risk) with tomatoes and lettuce. I think it's $7 or $8? It seems pricey, but portions are large. They also offer various cuts of fish and fried options, but this is by far my favorite.
They also sell a myriad of other raw and pre-prepared goods, including produce, chowders, local apple cider, pies, and of course, fish and other seafood. Things are expensive, but then again, this is Princeton.
I used to literally live around the corner from this place and Blue Point Grill. As a matter of fact, the owner was my landlord (hi, Jack!). I can testify that their truck does come in early every morning with fresh fish (good thing I'm not a light sleeper).
Tip: Get the fries for your side dish (included with a sandwich). The other option is slaw, but these are some tasty thin-cut fries!