Or it is is summertime, you may want to take the train. You can take the Septa train directly from the airport to 30th street station in Philly, then the "gamblers express" NJTransit down to AC. When you leave the train station, you are given a free Jitney ride to one of the 12 casinos. You also get a Jitney ride back home.

10 of the casinos are located on the boardwalk and its a couple of miles (I am terrible with distances) from end-to-end (Showboat to Hilton) but very walkable - even in one shot. Worse case you have the rolling chairs (let someone else walk for you).

Also, any cab ride within the city limits is a maximum of $6. So even the Hilton to Harrahs (Marina) would only be $6.