At 1,070 feet, Champlain Mountain is Acadia National Park's second highest peak (Cadillac Mountain sits at 1,528 feet). It's closer to the Mount Desert Narrows, and it provides much better views of the water. It also has the great bonus of thin crowds, which is very different from the packed Cadillac summit.
From Park Loop Road, access the Beehive Trail across from the popular Sand Beach and follow it to the Bowl Trail. You'll pass junctions for the Beehive and Gorham Mountain, but ignore them and continue until you reach the shores of the Bowl, a picturesque pond carved by glaciers. Relax here and then go left, following the trail around the Bowl.
The trail will slowly turn north and begin to ascend, running up the smooth granite of the mountain. To the west, Cadillac and Dorr will appear almost as impressive as the ocean view to the east. Continue up the rock, following numerous cairns and blazes. It's fairly straightforward until just under the summit, where some minor scrambling is needed. At the summit, experience Mount Desert Island in its full splendor, and take in the fantastic views.
Follow your steps back down the mountain, and make sure to keep good footing on the slick stone. This hike is best attempted in dry weather; any moisture makes the granite as slippery as ice.
Note that Park Loop Road is closed December through April.