Clothing/Shoes/Weather Gear: Be sure to bring warm clothing with you, even in the summer months. Just make sure you have a jacket and full length pants if you are hiking or boating. High winds at the top of the mountains can make your time spent there uncomfortable and short. And it is not uncommon for temperatures to fall considerably, especially if a downeaster weather front comes through. The photo is my freezing my arse off at the top of Cadillac Mountain on a nice sunny day in May.
Miscellaneous: You better pack some jungle-strenght Mosquito Repellent, you're going to need it especially when camping. Do and you will be fine, though. I had just one, on my way back from the shower to our tent on the campground!
Clothing/Shoes/Weather Gear: Glad I had along my zip off shorts/pants. Came in handy in the changing weather. We had a perfect day in mid August to see Acadia National Park.