I visited Isle Au Haut in 2002, and like any island in New England, it was gorgeous. They have copies of the NPS trail map on the mail boat (a pdf of the trail map can also be viewed here:
http://www.nps.gov/acad/maps/IAHmap.pdf). Due to the island's size and length of the perimeter road, bringing a bike is a good idea. The island's village is clustered on the northern end of Isle Au Haut, while the rest of the land is pretty much forested (with the occasional house here and there). Most of the trails are concentrated around the Duck Harbor/Western Head vicinity in the southwest corner of the island, which is also where the park's campground is. These trails reminded me somewhat of the paths on Monhegan, with their ocean views and how they wind through the quiet woods and along the rocky coast. The park's headquarters is actually on the other side of the island, just to the southwest of the village, but there's a trail leading from there that goes into the woods, then turns right (south) to connect with the other trails toward Duck Harbor. At the southeast corner of Isle Au Haut, by Eastern Head, there's a sign on the perimeter road pointing the way to "Thunder Gulch." It's actually a smaller version of Thunder Hole (which you will have all to yourself) and it's well-worth the side trip. Further, the high point on the island is Mount Champlain (543'), which can be reached from an unmarked trail off the perimeter road, just east of the village. It's depicted on the Delorme map, and it's the only trail on that side of the road in the vicinity. Once reaching the Champlain summit, it appeared to continue on as a herd path to the park trail that starts behind the headquarters building. Notwithstanding the trail description in Delorme, I don't remember there being much for views on Champlain (but my memory could have gone fuzzy on the point over the years), but it's still a high point and thus still worth the visit.
Just overall, it's a very lovely island that is well worth the day trip. Spending a night or two there would further enable you to relax and leisurely enjoy the placid beauty of the island.