I can add a few things, as ive done the Boston drive a million times! Dont go inland, its really not worth it with such short time frame.
Lets hit some of the beaches real quick, just incase the weather is nice.
- As stated above, good beaches in York, grew up going there every summer as a kid.
- Higgins: outside of Portland, ok beach
- Popham State park: check the tides as its great for kids in the little tide pools when tides out
- Pemaquid Beach: nice little beach but might not be worth the drive off route 1 unless your going to see the lighthouse
- Birch Point: outside of rockland, very tiny little beach, nice spot if your in the area to do Rocklands breakwater walk
- Lincolnville: to the right of the ferry terminal is a nice spot and you will be driving right past it
Acadia is great, and lots to do, but tourists are mad up there and expected to be a lot more this summer from what ive heard. My favorite is Camden as its like a micro Acadia, with lots of trails and a great lake to go swimming at, and plenty of schooners to take for a day sail. I'm a little biased as its where I live
Anyway, my quick run down would be
Portland (stay over), Freeport (passing through), Boothbay (stay over), Pemaquid (passing through), Camden (stay), Belfast (lunch stop), Acadia (stay a few nights)