Before you leave the Thomaston area, head north just a few miles to Camden and spend a day or two at Camden Hills State Park and the surrounding area. If you like hiking, the state park has a number of trails up coastal mountains with great views. I like Bald Rock Mountain and Megunticook Mountain, and the trails up Maiden Cliff are spectacular but the view is over inland lakes, not the coast. If you want the short and easy version, drive your car to the top of Mt. Battie for a truly spectacular view of Camden Harbor and Penobscot Bay.
In the region you'll find a bunch of restaurants and shops in Belfast, Camden and Rockland. Ducktrap Bay Trading Company in Camden has nice decoys (bring your wallet!) and Ducktrap Decoys up the road in Lincolnville (just the other side of the state park) has some too. You have your pick of food, from nationally recognized gourmet "farm to table" (Primo in Rockland, again, bring your wallet) to lobster rolls on the dock. (Take the long drive down from Thomaston to the end of the peninsula at Port Clyde and check out the Port Clyde General Store and Dipnet Restaurant.)
If you want to get out on the water, or if you are into birding or painting or just great sightseeing, take the ferry from Port Clyde about 15 miles out to Monhegan and spend a day hiking the trails on the island and visiting the many artists studios.
If it was me, I'd stick to the Camden region and avoid the longer drive up to Acadia National Park. Acadia is gorgeous, but getting out there in the mid-summer tourist rush can be frustrating. But the mountains are bigger than the Camden Hills, and it is spectacular. Two ways to avoid the crowds up there are to take the ferry out to Isle au Haut or drive up to the Schoodic Peninsula. Both are parts of Acadia, but much less visited than the parts of the park on Mount Desert Island. If you go up to Schoodic, Stonington is great little fishing village worth a visit.
If you are thinking of the Canadian route home and want a very pretty drive with maybe some brook trout or landlocked salmon fishing, head up either Route 201 to cross the border at Jackman or Route 27 to cross at Coburn Gore. Either route will take you through some remote timberland, and I'd be happy to provide some of my brook trout spots via pm. (The better ones are mostly fly fishing only.)
Or, it you are headed over to NH and Vermont, head west on Route 2. It's the quickest way over to Montpelier and Burlington and will take you through some very nice Appalachian scenery in western Maine and New Hampshire. For a longer but more scenic route, head north on Route 26 from Bethel, up through Grafton Notch State Park (mountains, cliffs, waterfalls, the "toughest mile" of the Appalachian Trail), and back down to Route 2 after passing through Errol and Gorham, NH. Lots of fishing on that route, too. Or a longer detour would take you up Route 17 to Rangely, Maine, one of the nation's oldest destination fishing towns, where you can find great fishing for landlocked salmon and brook trout, and great views. The "Height of Land" view over the Rangely Lakes is worth the drive by itself.
Oh, and take the ferry across Lake Champlain instead of driving around. Definitely worth it for the Green Mountain and Adirondack views.
Once you get out of northern New England, someone else will have to help you.
Here's hoping the pup doesn't get carsick!