Can I book my return flight to another airport? (Open Jaw ticketing)
 

I am flying to Miami and would like to leave out of LGA and return to EWR. How can I do this? Can I do this online?

Thanks!

This is called an Open Jaw ticket and is no problem as long as the open jaw distance is less than the distance of either of the two flown legs. Use the multi city option on dot com. Pricing normally is pieced together using both legs.

Sure. go to the AA website, choose the booking screen, then, below the boxes, check "Advanced," followed by "Multi City. Segment one will be (in your situation) LGA to MIA; and Segment two will be MIA to EWR. Easy Peasy

Sure you can. I think you can just put New York City in the search box instead of a specific airport to show all the NYC airports (JFK/LGA/EWR) but if that doesn't work you'll need to use the "Include nearby airports" option from AA's advanced search page. Just chose a return to EWR instead of LGA.

edit: You can put NYC in as the origin and MIA as the destination and it'll let you book a round trip using flights between the NYC airports and MIA without having to do the multi-city thing.

Since LGA and EWR are co-terminals, it should not even be necessary to use the multi-city option to book the OP's ticket. Search for a round-trip from "NYC" (all airports) to MIA.