I'm not entirely sure about accommodation, but I can give you some tips on things to do!
The biggest beach in the Portland area is Old Orchard Beach, which gets pretty busy in the summer months (tons of French-Canadians on vacation!). There's a pier, arcade, and a little amusement park as well if you're into that. If not, there are several smaller beaches along the coast south of Old Orchard (Kennebunk, Wells, Ogunquit, York). However, keep in mind that the ocean water in Maine is rather cold (around 60 degrees). Not unbearable but it takes a little while to get used to.
As for antiques and main street shopping, you're in the right place. Pretty much every New England town has at least one antiques store. My favourite part of Maine, and one of the quaintest areas, is the mid-coast region, particularly the towns of Wiscasset, Boothbay, Damariscotta, and Camden. You *cannot* pass up a chance to visit this area - plenty of quaint little antiques stores and beautiful scenery, particularly at Pemaquid Point in Bristol (south of Damariscotta) and from atop Mt. Battie in Camden (no need to hike - you can drive up it!)
As for more natural scenery, if you're willing to drive further east, Acadia National Park is always worth the visit. It's usually crawling with tourists, but the view from Cadillac Mountain is stunning. Another option is to go west into New Hampshire to see the White Mountains (you wouldn't regret it!). The Kancamagus Highway, Franconia Notch, and the Mt. Washington auto road offer some of the most spectacular scenery east of the Mississippi.
Every tourist visiting northern New England wants to see a moose, but unfortunately many of them leave disappointed. Don't get your hopes up, because your chances of actually seeing one aren't great. However, the best advice I can give is to go as far north as you can, onto the back roads. I do know of some tour companies (in Lincoln, NH for example) that offer day tours with guarantees of moose sightings, but... it might not be worth the money.
Wherever you end up, you should probably ask the locals if they know of roads with frequent moose sightings. Be careful, though - you don't want to hit one of those things.
Oh, I forgot to mention lighthouses! The Portland Head Light in Cape Elizabeth (just south of Portland) is beautiful. Another popular one is the Nubble Light in York, as well as Pemaquid Point. I also wanted to mention Portsmouth, New Hampshire, which would make a great stop - a nice place for main street shopping, and the Strawberry Banke area is a fabulously preserved colonial neighbourhood. Very beautiful.