Mount Desert Island is a pretty packed place in the summertime. Bar Harbor will be wall-to-wall traffic. I hope you've already made your camping reservations! Just west of Mount Desert is the peninsula with the towns of Blue Hill, Castine, Stonington, and some little hamlets. It's much more of a "Down East" experience. Also, there's a beautiful little Nature Conservancy property in Sunset, a stone's throw from Stonington, called Barred Island. Totally moss-covered and a pretty little beach at the end of the trail. Right next to it is Goose Cove Lodge, the perfect honeymoon retreat, if ever there was one!
On the way to Acadia is Pemaquid Light and Pemaquid Point; classic Maine photo locations, particularly at sunrise. Stop in picturesque New Harbor for lobster on the pier on the way down the peninsula to Pemaquid.
Further eastward along the coast are all sorts of small towns, many of which have seen better days, but still very quaint. Jonesport harbor is a great pano shot, especially if all the fishing boats are tied up. Try to reserve a campsite at Cobscook Bay State Park; beautiful and VERY popular. It's near Reversing Falls State Park. Make reservations now.
Pick up the DeLorme Maine Atlas and explore as many back roads as you have time for.