Miss Portland Diner.
Plan a day in Bath. Drive by the huge Bath Iron Works, see the Maine Maritime Museum, go to the Kennebeck Tavern, take a cutter out of the Museum down Kennebec River to the outlet and explore Georgetown if you have the time.
End of August is about the best time to visit Old Orchard Beach. Water temperature usually reaches 50 degrees by that time!
Portland Head Light, outside the city of Portland.
Kennebunkport: One of the most charming towns on the southern Maine coast. There are plenty of great restaurants, hotels, historic inns and shops. It's pretty busy during the summer.
Do *not* miss driving / stopping by The Bush compound on Walker's Point . Keep driving along Ocean Avenue for great ocean views and jaw-dropping seaside mansions.
The Colony Hotel: A beautiful, historic grand hotel built in 1914. You don't have to stay here to stroll the beautiful grounds, and poke around the first floor of the hotel. You can eat in the Marine Room downstairs, by the pool. The Colony serves an awesome breakfast every morning in the Grand Dining Room. There is also a Friday Night Mariners Buffet.
Other great Kennebunkport restaurants: The Clam Shack, Mabel's Lobster Claw, Pedro's Mexican Restaurant, Old Vines Wine Bar, Alisson's Restaurant & Pub, Hurricane Restaurant, The Pilot House, Ocean Cape Arundel Inn (expensive), Arundel Wharf Restaurant and The Spirit of Massachusetts Restaurant (floating restaurant on a sailboat).
Cape Porpoise: The Ramp Bar & Grill (excellent food! Don't be deceived by its looks), Pier 77 Restaurant, Nunan's Lobster Hut.
York Beach: The Goldenrod Restaurant on Short Sands Beach (since 1896) - fun especially for families.
Good brewpubs in the area: Gritty's in Portland, Federal Jack's Restaurant and Brewpub in Kennebunkport, Sea Dog Brewing Company, Topsham.
If you enjoy baseball, you can visit Hadlock Field in Portland, home of the AA affiliate of the Boston Red Sox: The Portland Sea Dogs. I think there's a baseball team and field in Old Orchard Beach as well.
Beware: There's a 'Little Somalia' a few blocks from Hadlock Field. We've been going to Portland for decades - so far we've had no trouble.
Other favorite spots of ours along coastal Maine:
Ogunquit / Perkins Cove / Marginal Way (can be very crowded and touristy in summer). The historic Ogunquit Playhouse is a great place to see a "summer stock" play or musical.
Harpswell / Orr's Island / Bailey Island (SE of Topsham and Brunswick). Usually quiet... rustic and restful.
Boothbay Harbor
Bar Harbor and Mt. Desert Island -- We love the beautiful, rustic Cranberry Isles - accessible by boats that also deliver U.S. Mail to island residents.
Have fun...