Dnvrfox:
I'm sure PastorBob or CapeJohn will have much better information that I do. However, my wife is from the Easton/Bridgewater area and we have been in and out of Boston many times.
I don't see anything wrong with the plan to buy a couple of Wal Mart bikes for the brief touring you plan. Its actually a really good plan. When I used to do a lot of oil field work away from home I did the same thing a couple of times (Calgary, Williston,), and then just gave the bike away.
You might think about renting a minivan and then both bikes will fit inside the van. With four boys, our family always looked like Wringling Bros, Barnum and Bailey when we traveled so the van worked good for us.
I've never toured up there, but have been up into NH and Vermont (two sisters live up there) often, as well as Maine. Beautiful country. It's easy to understand why the Pilgrims settled in Massachusetts rather than southTexas.