This one requires some time commitment if you are coming from Southern Maine, but you can find big adventure with minimal effort here. Camp at Lily Bay State Park, then drive to Rockwood where you can catch a small ferry to the island in the middle of Moosehead Lake. The lake is notoriously choppy, so the ferry ride can be a thrill all by itself. It is possible to canoe/kayak to the island, but the conditions are often a little rough, so be prepared.
Once on the island, the hike to the peak of Mt. Kineo is totally achievable for young hikers, and there are nice views along the way. Not much view from the peak, but there's a fire tower at the top that the brave among you can climb. We did this with kids 9 to 11, but it's well within reach of the younger crowd, too.