There's plenty to see in Boston; besides the historical sites, the Science Museum is good, the Aquarium is good, and there are a few quite nice art museums, among other things.
Do be aware that propane is not allowed in the Boston area tunnels, even if turned off at the tank. Driving anything around Boston streets is no fun at all; the subway, etc. is far less harrowing.
Philly has many many cool things to see if you've been convinced to avoid the NJ coast. The Art Institute is fantastic, one of the very greatest art museums in North America; and the historical sites are worth visiting, and lots more. Driving through Philly is no treat, either, but IMHO not quite so horrid as Boston.
