Love Acadia! We've been camping there twice.
Make sure you see the "quiet" side of the island (other side from Bar Harbor). Truly beautiful.
Have a lobster roll on a pier...any pier.
One thing we really enjoyed was a sunrise kayak trip on one of the lakes (can't remember which one). Great, low-impact environmental way to see the wildlife and have some fun.
Bar Harbor is also fun for a night out...good restaurants, fun shopping, usually some live music in the park.
Have a great trip!