Renting a car is a good option...you should be able to do it cheap. I doubt you can return it same day in AC though but I mean..its prob like 20 a day and will be convenient to have (though its easy to get around w/o a car also).
There is a train that runs from Philadelphia International to 30th Street Station (via Septa). From there, you can transfer to a NJ Transit Train to AC. Go to
www.septa.com and
www.njtransit.com for schedules. The rental car is fastest route though...about an hour to AC from the airport.
Jeff