Some day, I would like to very much ride the Mass Central Rail trail. Alas, due to the political posturing and "Nimby" attitude of certain Massachusetts towns, it does not look like it will ever be built in its entirety.
The original plan was to have it begin in Waltham, at the Linden street bridge, and continue all the way to Northhampton.

Nice pic of the cape cod rail trail.

A few times in the past, I have ridden on the Provincelands bike trails, Provincetown MA. I won't put these on my list. For the un-familiar, these were built in the early 60's, and have a lot of steep grades with bad, frequently blind, curves. National park personnel deal with several bad crashes per summer, many involving tourists. As Steve Faust put it, the person designed these bike trails, and the person who designed the wild mouse amusement park ride, are probably the same guy. Link to article, with pics