Cruising the Rhine River is one of the best ways to see this amazing region of Germany. Cruise boats stop nearly every hour at many places up and down the river. You can use a Eurail Pass to access the cruise boats at no additionaly cost. The area stretching from Mainz to Koblenz gets most of the traffic since it is the most romantic section of river. You can actually make this whole stretch of river in 5.5 hours if going downstream and 8.5 hours if going upstream. Of course, it would take longer if you make stops at the towns and sights along the river. Along the river you will see old fortresses, castles, vineyards, and boats. If time is a problem, you don't have to do the whole section from Mainz to Koblenz - in fact, most tour guides recommend that you just pick a section and only focus on it. Rick Steves suggests the section from St. Goar to Bacharach if you want the most scenic hour. Marksburg Castle is not far from Koblenz and is considered the best preserved of all Rhine castles. The Rheinfels Castle (pictured below) near St. Goar was once the mightiest castle on the Rhine River. It is relatively ruined now but is one of the neatest places to visit on all the river.
Looking at Rheinfels Castle from the Rhine
Some of the towns along the river are extremely picturesque and fun to visit. They abound with history and scenery. One such town is Bacharach (pictured below.) Both Bacharach and St. Goar are nice villages to stay in for a night.
Bacharach - most beautiful town on the Rhine