In the peak periods, over 25,000 shorebirds may be seen in a single 3-hour outing, of more than 20 species. Harvard, MA Best time of year for Oxbow is May-June. You can see parts of this refuge from Route 2 East, large swamps with herons nesting in trees. I've been there several times so check out my trip reports page. Winter birding is good for gulls, ducks, and eagles and I've also heard of Snowy Owls being sighted there in the middle of Winter.
Many migrants pass through during spring and fall, and hence Plum Island is recommended year-round.
To get to the Parker River Wildlife Refuge, take the Route 113 East exit off of Route 95. 113 will change into 1A in Newbury, from there you should be able to follow signs to Plum Island and the refuge. Also in Newburyport, Water Street runs along the Merrimac river where there are several good spots to pull over and birdwatch. Its that BIG lake in the center of the state. I've heard this site mentioned on many occasions, but its so large that one could probably visit it several times and not see the same areas twice.