Not everyone who visits the area is aware of it, but Bar Harbor does have an airport, but not in Bar Harbor, not even on Mount desert Island for that matter. The Bar Harbor Airport is located in Trenton, Maine.
That may be a good distance away, but you can still watch planes land and take off while in Bar Harbor - if you know where to go.
My favorite place to go just before dusk to watch planes land and take off is to hike up to the summit of great Hill. Once there you can sit down on the bald summit and look out across the darkening sky toward Trenton and watch as approaching planes circle the airport and come in for a landing. It's the best night time view on the island and your surrounded by stars above and complete silence.