There are great suggestions here and you can't go wrong with any of them, IMO.

In case you are a pizza fan, I will add Pizzeria Regina. No, it is not a place for a romantic dinner or formal meal... not at all. But, it is a Boston landmark, amazing pizza, and a not-to-miss spot in the North End. Don't go if you have only one night to eat there but if you have multiple nights and enjoy a good pizza, put it on your list.

If you go to dinner in the North End, make the effort to walk to and from the restaurant. This neighborhood has a special charm and Old World quality to it that you can soak up on a good, leisurely walk. Maybe stop for a desert or pastry on the way home!