Lovely trip! Can I come? 3-4 weeks is a long time. What are you all interested in? Re shopping - think luggage allowance!!
Boston is lovely - lots of great museums - Museum of Fine Arts and the Isabella Gardiner. Try a Red Sox game if the right season. I loved doing the Freedom walk around the city. Lots of super places to just mooch. I am not sure Filenes basement is there anymore.
There are plenty of shopping outlets but my American friend always drives to Kittery in Maine.
Do look at cape Ann, smaller and cuter than Cape cod.
Lots of history in Mass so plenty of driving tours.
Whale watching can be fun - if you are a super sailor!
Rhode island is great with the "cottages" - think Gatsby and all.
Florida - I love Miami and Disney of course but a week doesn't seem enough. Perhaps slightly more time there.