What to do during a 7 hour layover in Newark, NJ?

My mother and I have a layover in Newark, NJ this june. I am interested in knowing what we should do during this time. Should we go into NYC or just stay in Newark? If we stay in Newark, what should we do? Is there good things to do in the airport? We want to spend our money in Greece, not in Jersey, so whatever your suggestions please consider to have them be frugal. Thank you so much!