Kids will love Fort Knox I bet. Bring a flashlight.
Since you mentioned off the beaten path places, I highly, highly recommend you take an afternoon or something to schoodic point. It's part of Acadia NP, but not on MDI.
Miles of beautiful pink granite beach, far less crowded than Bar Harbor and MDI. The waves typically crash very high in the air. Very scenic, often quiet place. Bring sammiches or something to picnic there.
At Camden state park, Maiden Cliff Trail is a short but very rewarding hike. And on the rte 1 side of the park, I would definitely recommend driving up mount battie for a quick look around. Nice view of Camden harbor and you can see out to MDI and Isle au Haut on a decent day. You could probably find a couple wild blueberries to eat up there.
The breakwater and lighthouse in rockland is very nice.
The lighthouse and park at Owls Head is a favorite of mine.
Maine Military Supply is in Holden.(thanks, jlr99) Might be worth a visit. Largest mil-surp store in New England (I think) and I believe they have a range and full auto rental now. I haven't been there in a few years so I can't say for sure. Check their website maybe.
There's a giant beer wine liquor store in Ellsworth. Oh, that reminds me, there's an Irish pub/ restaurant right on the main drag in Ellsworth called Finns. I love the food there.
Unlikely to see a moose in the areas you listed, but certainly possible.
Are you driving up or flying in?
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