If your flight is on time, you can take the airtrain to the railroad station (10 min) and take a train to Manhattan (22 minutes). Depending on how long you have to wait for a train 0-30 minutes, you'll be there by noon. Take one of the following trains back and you'll have plenty of time to make your 7:15
LV ARRIVE
A159 5:05pm 5:27pm
7261 5:07pm 5:30pm
7861 5:14pm 5:37pm

That gives you enough time to go to the Museum of Modern Art (MOMA) and walk around Fifth Avenue and have lunch, or go downtown to Wall Street and South Street Seaport, shop at Bloomindales and the Apple Store, or whatever you feel like,