It's too bad you aren't staying some place right in the village. The Holiday Inn is a bit of a walk, along a busy highway, to get to the village. Any chance you can change your reservation to one of the village Bed and Breakfasts? Aysgarth Station has a room available for all but one night of your stay. Jane could possibly suggest some place for that one night.
One thing to do, for sure, is take the Oli's Trolley tour. That will get you into Acadia National Park, along the coastal scenery and to the top of Cadillac Mountain. The free shuttle buses are no longer running. So Oli's Trolley or a taxi ride tour will be your only way to see all of the Park. There are some nice walks you can take right from the village, depending on how good a shape you are in. There is a trail into Great Meadow from which you could also climb Dorr Mountain. There is a trail at the far end of Main Street that will take you all the way to Schooner Head Overlook from which you can also hike to Sand Beach. That's about a five mile round trip walk from the village center to Sand Beach.
As mentioned, the village is perfectly safe to walk around in at night. But you might want to get a taxi to take you back to the Holiday Inn. Not because it's unsafe, but because it's a long walk in the dark.
In the village, there is also the Abbe Museum, which shows Native American artifacts. It's quite nice. I'm not sure what days it is open at this time of year. On Mt. Desert Street, where the Abbe Museum is, there is also a very nice church with beautiful windows that is open to tour in the mornings, St. Saviour's. And the public library across the street from the church is worth a visit just because of it's beautiful interior architecture.
Also in the village is a nice half mile walk along the shore, with the islands to one side and the old mansions to the other. Just go down to the water at the village park and you will see the path going off to your right, in front of the Bar Harbor Inn and then on.
At low tide you can walk down Bridge Street and across the tidal sand bar over to Bar Island. Nice views back toward the mountains of Acadia and rather neat walking on the exposed ocean floor.