Cadillac Mountain is actually located inside Acadia National Park, but I think this noteworthy landmark deserves its own entry, especially since we had quite a noteworthy experience with this giant. There is no special admission price for the mountain, but to get inside the park, you have to buy at least a 5-day pass, which runs $20 (there's no single-day entry).
Cadillac Mountain is the highest peak on the Easter seaboard, the first place in the USA that the sun strikes each morning. From the top, which has been made easily accessible to the public thanks to Acadia National Park and Reserve, provides breathtaking panoramic views of Bar Harbor and out to the ocean, including the interesting Porcupine Islands. When I visited with my family, we actually had to take the drive up twice in one day - our first attempt, near lunchtime, found us in the middle of a fairly intense rainstorm with clouds completely obstructing any view. From down below, the storm had not appeared to be very serious, but we found ourselves in what seemed to be the center of a rain cloud that had settled itself on the peak. We tried to wait it out a bit, but the weather was so severe that we gave up any hope of being able to experience the summit that day. However, we learned a little something about the fickleness of the weather in Maine. By late afternoon, the fog had lifted, and since it was our only chance, we decided to give it one more shot. This time, the place had cleared out and even dried up already, giving up the magnificent panorama the mountain is so popular for. Even if we did have to try twice, it was worthwhile. We even found the geological summit marker, which is quite a ways from the popular tourist area.
To get the most out of the mountain, before you start your trek up to the summit (which is quite a trek), be sure to take a look at a map of the park. Food establishments are quite spread out, and the small shop on Cadillac Mountain only sells limited snack items. It might be nice to take a picnic with you. Be aware that the drive around the cliffs and up the winding hills (which is a gorgeous one) will take you about 15 minutes without much traffic, even if you are already inside the park and close to the entrance for the road up the mountain. Also, be ready for wind and cold on the summit. Even if there's blistering heat down below, this altitude is definitely in sweatshirt territory. Also, remember that Park Loop, where you'll probably start from, runs one way for a good while, so be sure you can get back where you need to be. Give yourself plenty of time and be ready to appreciate the natural beauty of the Maine coastline.