There is little to say about Katahdin. It is far from easy to get to the top (about a 5 mile hike each way with an elevation change in excess of 4,000 feet), but the views are stunning. It's the northern terminus of the Appalachian Trail for a reason. The through-hikers at the top alone can be worth the trek with their exuberant joy after finishing the 2,200 mile journey starting in northern Georgia. If you don't have time to make the hike, Baxter State Park is an excellent trip none-the-less, with a variety of smaller mountains and shorter trails to explore. Campsites in the park go quickly though so make sure you get reservations well in advance.