Three areas of interest...
1. If the 22s are in use, constitution beach. Get off at the Orient Heights or Woods Hole station on the blue line at the T and walk down bennington St until you get to the beach overpass. Need at least a 70-300 if you want to get anything smaller than the big heavies.
2. Castle island in south Boston. You need a car for this.
Both of these spots are useless in the morning, the sun is in the wrong position and you can only use them if the 4s/22s are in use.
If rwy 27 is in use there's a park in Winthrop that I like to spot at and it's good through the day. Never tried in the morning though.
If the 33/32 are in use, you are out of luck, unless you have a boat
... unless you want to try the parking garage, and then you're at risk of being tossed by Massport.