I love Acadia! I went last year and it broke into my top national parks list. Beautiful park, wonderful camping experience. Only bad part is how out of the way it is (which is also a good part)
If you're into hiking, there are so many great places to hike there. We did the Gorham Mt Loop, the Beehive Loop, and a Mt. Cadillac sunrise hike, and all of them were incredible.
If you're into biking, there's the carriage roads on Mt. Desert Island, and if you're willing to go a bit out of your way (perhaps on the drive up or down the coast), Isle au Haut is a fantastic little island to rent a bike and ride around on (but bring bug spray!)
If you're interested in a more relaxed experience, there's puffin tours, the flat and calm Jordan Path Pond and neighboring Jordan Pond House, the Wild Gardens of Acadia, Sand Beach (water is cold though!), and Bar Harbor is also just wonderful to visit and walk around.
While you're up in that area of the world, be sure to visit the Bioluminescent Bay in Castine Harbor and do the kayak tour! I've been to the bio bays in Puerto Rico (which claim they're the "only ones in the world"), but they have nothing on what I saw in Maine. Truly magical experience.