I'll do a third to Acadia National Park, it's just plain beautiful. It's on Mt. Desrt Island, along with Bar Harbor. The island has some great two lanes which won't give you much speed, but great touring and views, and lots of distance to cover. Some great lobster dinners in the small towns around the island.
Cadillac Mountain (drive up) has fantastic views on a clear day, and sunrise can't be beat.
Sand Beach in the park is great. No sand, just ground up shells that look like sand, but good for laying out in the sun. The water lends new meaning to cold, even in August.
If you bring the tent, there are some nice campgrounds on Somes Sound, the only fjord in North America.
Great places to eat in Bar Harbor. The sunset cruise is good too, great scenery plus cocktails.
Check out
www.acadiamagic.com for a good overview of the area.
Other good stuff in midcoast Maine:
Owl's Head Transportation Museum, just south of Rockland. Old cars, planes, etc.
Camden is a great town...lot's to see, more lobster.
Freeport's got the LL Bean outlet, if someone in the party has a shopping urge, and you don't mind busy.
Did I mention lobster?