Eating off campus is a good option when you need something different than the food offered on campus and you can't get home for a home-cooked meal. If you need something quick, there's a lot of fast food places nearby, like Wendy's, McDonald's, KFC, and Burger King. If you want to sit down and possibly spend a bit more money, Applebee's and Olive Garden are good places to go. Applebee's has a two for $20 special where two people can get two meals with an appetizer for $20, so you're getting plenty of good food at a good price—which is key on a student budget.
There are also plenty of restaurants in Princeton if you have a car (or if you attend the campus there). Overall, there are plenty of off-campus dining options in the area, regardless of your tastes or dietary specifications.