Princeton University has a kosher dining hall if you go while school is in session.
If not, there is a Shop Rite Kosher Experience in Lawrenceville. Shop Rites usually have sandwiches, rotisserie chicken and that sort of stuff and they are located in the Mercer Mall which should be relatively easy to get to from Trenton. (I have been to the one in Cherry Hill but not the one in Lawrenceville.)
There are also McCaffrey supermarket in both Princeton and Yardley that have kosher products.
Look them up on Shamash.
The distances in NJ are not as far as they look. Small towns in NJ tended to incorporate as separate towns where elsewhere they would be neighborhoods of a city.