The Cape Cod Canal Bike Trail tends to play second fiddle to its big brother, the Cape Cod Rail Trail, but having now done both, I wouldn't sell the Canal Trail short. It's a pleasant 6 1/2 miles from the Canal Visitor's Center (the eastern end of the trail) to the west end, down past the Bourne Bridge. On the day I road it, the best direction to do it was from the Visitor's Center to Bourne and then back. The ride out had rather stiff head winds, which kept me cool, but slowed me down quite a bit -- it took me forty to fiftty minutes to get to the Bourne end and only about twenty minutes to get back. It was great to have the wind at my back for the return trip.
The Canal is always visible from the trail (you're right up again it) and you have great views of boats as you ride along. I didn't see any large ships in the Canal, though they do use it.
But the best reason for starting the circuit at the Canal Visitor's Center is the Center itself. It a pleasant place to spend a half hour (especially handy if you are waiting for someone to pick you up, as I was) with a good deal of information about the building of the Canal and the running of it (including some computer displays that let you zoom in on traffic in the canal).
There's a gift shop (mostly books) and quite a bit of free material about the Canal (including a couple of maps) and the general area. There's also a pleasant porch with a few rocking chairs; a great place to sit and watch the traffic on the Canal.