Exactly where you’re leaving from to get to the airport can make a big difference. I suggest you figure out where you’d like to have lunch — as you’ll need to head to the airport right afterwards — and decide on that basis.

I think the Pub Room at Keens is a great place for lunch. From there you could walk to Penn Station to catch the train to Newark. (To be honest, from there you also can catch the F train subway to the bus to LaGuardia. As long as you leave Keens by, say, 1:30, you’ll be fine with either option.)