Acadia/Bar Harbor definitely but it would be better to stay there-it's much easier than the hour or so drive from Bangor down. If "late summer" means after Labor Day I wouldn't worry too much about reservations. Before then definitely. Try the Bar Harbor KOA.
Brunswick will give you good access to Freeport-shopping and the Desert of Maine. Very interesting place. Quite strange to see a desert there. You can also access Portland fairly easily. Sailing, whale watches, lots of shops and restaurants. There are also a number of museums there. If you don't mind driving a bit you can hit "quintessential" New England in Kennebunkport and Ogunquit. Kennebunkport has shopping, restaurants, sailing and beautiful ocean views. Ogunquit has one of the most beautiful beaches in ME but be prepared to see a LOT of "alternative" lifestyles. It's a very family-oriented area but does cater to many different types of people. It's one of our favorite places.
There's antiquing almost anywhere in Maine.
I'm not sure what "bang for the buck" you'll get using Bangor as a base. Maine's a fairly good-sized state. You don't say how long you have but I think you'd be better off to move around a bit. The southern beaches are quite nice although crowded before Labor Day. A few more hints of how long you have and what you like to do might help.
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[purple]RV-less for now but our spirits are still on the open road. [/purple]