I covered a few bits of the Herengracht as part of a 'canals walk' out of my Eyewitness Guide and it's got the showiest Golden Age houses on it of the main canals. It also seems to be the longest and I have to admit that I didn't just go from one end to the other. If you get that urge it runs from the station all the way out to the Amstel, then an extension loops past the Hermitage and the Jewish quarter to the Maritime museum and out to the IJ - plus lots of architecture to look at on the way.
Cafes/bars along the way for refreshment and people-watching.