The regional train is the backbone of travel in the valley — it stops at all the main towns on the left bank and runs every 30 minutes. Tickets are inexpensive and the journey is scenic. Combined with the cross-river ferries (which run regularly at St. Goar/St. Goarshausen, Boppard, and other crossings), you can move freely between the left- and right-bank towns without a car.
The river boat (Köln-Düsseldorfer) is the most atmospheric option between April and October. Buying a one-way boat ticket and returning by train gives you the experience of the river without the time commitment of two boat journeys.
Cycling through the valley is one of the best ways to see it at close range. The cycle path along both banks is well-maintained, and regional trains accept bicycles. Rental is available in the main towns.