Massachusetts vacation ideas?

Fall in New England is beautiful- it's my favorite time of year to visit home.

While I love the Cape in the fall, be warned that not everything will be open.  It likely will not be very busy, though there will be people there.  When you say "beach," do you mean actually lay on the beach and swim?  It won't be the right weather for that.  Fall is a good time for walking the beach wearing warm clothing, looking for shells, etc.  I doubt anything will be happening this year, but you can check to see if any of the fall festivals are actually going on- there's usually a cranberry festival in the Wareham area (right before the Cape bridges) and usually a festival in Yarmouth over Columbus Day weekend that includes fireworks on the beach.  However, I am doubtful that any of those will actually be held this year due to the pandemic.

If you bike and you're heading to the Cape, I do recommend checking out the Cape Cod Rail Trail.  I love this in the fall.  You'll ride past several different kinds of areas, including cranberry bogs.

Other Cape ideas- Cape Cod Canal (biking or walking), Truro Vineyard (far though, but offers a great wine tasting and they are currently open), Chatham fish pier (watch the boats come in and unload, watch the many seals, get takeout from the fish market), the National Seashore (depending on what's open.)

Please be aware that Salem has canceled many of its typical October events and may not have many of the sites open due to the pandemic.

If you have never been before, I would recommend doing one of the typical tourist things... walking all or some of the Freedom Trail in Boston.  I found it tedious and boring as a child (when you grow up in the area, it's a common field trip!), but as an adult who has moved away, I miss being surrounded by so much history!

Other ideas...

If you're going to Salem, explore more of the North Shore area and maybe Cape Ann (though not sure what will be open/allowed in the fall.)

Because you mentioned a "walk in the woods," why not go old school walk in the woods and check out Walden Pond, where Thoreau probably did his best thinking?  It's a state reservation, so make sure it's open before you go.

If you have a full day, you might want to consider a drive out to western Massachusetts, to the Berkshires (gorgeous fall out there!)  Some of the ski resorts run scenic chair lifts and alpine slides in the fall, though I'm not sure if that will happen this year.  If you're driving from western Mass to eastern Mass or vice versa, check to see if the Big E drive-through bakery/food event will be running in Springfield.  The Big E is a huge New England fair that happens every fall, but has been canceled for this year.  However, they've been doing the fair special foods on certain days in a drive-through bakery kind of format.  I don't know what dishes they're serving, but the Big E is known for having a building of each state (not just dedicated to the state, but the building itself is considered to be in that state, so even though the fairgrounds are in MA, you can visit all six New England states in less than ten minutes), and each offers a food from that state.  If you're driving through anyway and the food event is running, might be worth a stop!

You could also head up to the White Mountains area of New Hampshire.  The Kancamagus Highway is a scenic route through the area.

In areas away from the major cities or Capes, visit a local apple orchard.  I have lived outside of New England for over 15 years and still think New England apples are THE best.  Ordinarily, I'd tell you to check out any area's local social media or website to find out about fall festivals, but I'm assuming none of those will happen this year.

Take a drive up to southern Maine.  Visit a rocky Maine beach (York is about an hour and twenty minutes from Boston.)  Get some seafood.

Do keep up with current restrictions and double check as it gets closer.  Right now, I believe most of New England is okay to travel within from the New England states, though Massachusetts may still be restricting Rhode Island.

ETA: To address the quarantine comment in another answer, CT does not require people coming from EVERY state to quarantine (people from NH, VT, MA, ME, RI, NY, and NJ do not, as well as a handful of other states.)  Depending on the day, people from about 30-35 states need to quarantine.  Massachusetts usually has one more state than CT (CT took RI off the list recently, but MA kept it.)  Check the actual state you're visiting for the requirements.  Some states are accepting a negative test within 72 hours of arrival, but they usually require the PCR test and NOT the rapid, so keep that in mind.  Maine accepts the test, CT and MA have wavered on it and I'm not sure of their current requirements.  It sounds like the original asker has already looked into that and knows that the test is valid in lieu of quarantine.  My advice would be as I said above, just stay current on restrictions as they change frequently.

Dont go it sucks

Cape Cod traffic typically falls off after Labour Day weekend, but keep in mind that some restaurants and shoppes close by October.

If you are flying into Boston and you are a big history buff, then definitely do the historical stuff in Boston associated with the Revolution.  If crowding inside the buildings is a concern (or things are closed), then just walk the "freedom trail" from Boston Common to Bunker Hill.

Here in CT, right next to Mass, anyone coming from another state must quarentine for 14 days.This is a pandemic that will not be over by October.Nothing is open.