I went to Maine for the first time this past August. We ended up in Bar Harbor/Acadia for most of the trip and had a great time. I brought my bike and kayak. I could bike from either of the two hotels we stayed at and go right into Acadia. There is no real mountain biking in the park, but they have a huge system of carriage trails open to bikes that lead all over the park. I rode up Cadillac Mountain and passed all the cars near the top because of the traffic trying to get parking. The CX bike would have been perfectfor what I was on.Judging by the overall terrain, I am sure that you could find real trails around if you looked hard enough.
One hotel we stayed in was right on the water at the harbor. I took the kayak off the car roof and plopped it into the harbor. It was nice to take the kids around the harbor so easily, and the water was crystal clear. The other nice thing about that hotel is that we could walk to most of the restaurants and shops in town with the kids. It's a very cool area to check out, and I plan to return sometime. But, Bar Harbor is way up there and pretty far up from Portland I think. I have photos, but I guess they are too big to upload.