Hi gcInc:
When I applied for AP in person at Newark, they really did not want to know why I was planing to travel. I was asked why I did not apply by mail, and I just showed them my travel itenarary printout showing my flight booked for the following week, and said I did not have time to apply by mail. Traveling to visit the family is an acceptable reason.
Apart from the filled out form, photographs and the money, please take along your I485 receipt notice - they will not entertain you without that. Also, they do not dispense change at the cashier window, so bring the exact amount of money.