Plum Island is split between Newburyport, Newbury, Rowley, and Ipswich. Newburyport is best known as a seaport, and as shipbuilding center in days gone by. It's a pretty cool place to visit in its own right. The island is best known as an outstanding birding spot. It's a stop off on many many migration paths.
Birders come from all over the world to add to their life lists on Plum Island.
Recently updated info on the airport situation. Group Committed to Saving Historic Plum Island Airport So far as I know the airport is closed. The above links should have the latest info. Birds - See Also Parker River National Wildlife Refuge Birding America's Plum Island Page Fat Birder's Massachusetts Page A UK site with a wealth of info for birders planning a trip to Plum Island and other Massachusetts sites from abroad. Massbird.org Links to current reports on what birds are being seen on PI and elsewhere.