We have four days to spend between Bar Harbor and Boston ,leaving Bar Harbor on Sunday 7th October,arriving Boston Thursday 11th.We will be using public transport which limits our choice a bit.We had thought about Portland and maybe Old Orchard Beach!
Any ideas please?

Portland may be a good choice as it is a small walkable city. Lots of shops restaurants and bars.

Camden might be nice, but I'm not sure its that easy to get around without a car. but it has a lovely harbor, and some niuce sps and restaurants.

any reason you can't rent a car for those few days? you'd get to see a lot more that way.

oh, also Portsmouth NH. There would be a lot to see there in walking distance...historic homes, harbor. that would be a nice stop.

And you could also do Salem MA. Go to the Peabody Essex museum, and see some of the witch stuff.

Just curious and may be helpful for answers -- what is your "public transportation" from Bar Harbor to Boston. Train? Bus?

From Bar Harbor we get a Shuttle bus to Bangor.From there we can get a bus to Portland where we can get Amtrak or another bus to Boston!
We thought about Camden but to get there from Bangor by bus it seems that we would have to go to Boston first.
We will have come from Toronto by train to Quebec,then to Halifax.From there to Yarmouth & ferry to Bar Harbor.We'll return from Boston to Toronto(not sure how yet)to fly home to Scotland.A good round trip!!

It's a shame to miss Camden because it's beautiful, esp if you see windjammers sailing but the best view is taking the auto road up Mt Battie which overlooks the harbor.
Both Portland and Portsmouth are good choices. Both have boat trips you can take although I'm more familiar with Portsmouth and have enjoyed the harbor cruise and inland rivers cruise. They should have foliage cruises scheduled and I would expect you would need advance reservations. Because of the Strawbery Banke historic area, I would favor Portsmouth. Prescott Park on the water has lovely gardens and should still be gorgeous at that time of year. It's across the street from Strawbery Banke. Make sure you get a downtown area hotel and not one out on the traffic circle.

I would stay in Portland because there's lots of great restaurants, art galleries, etc. You can take a bus to Old Orchard Beach for a day trip. The beach is really nice there and there's a pier which has carnival-like attractions.

It would be excellent to rent a car and drive. There are so many beautiful seaside towns to visit along the way! And, the lighthouses! You can pick up literature with directions. Most you can drive right up to.

You could end at Old Orchard, which is not far (1/2 hour) from Portland, and take the train or bus from there to Boston (so as to avoid driving to Boston).