Favorite thing: We took a tour boat up the Rhine River from Koblenz to Mainz. Although the scenery was, of course, beautiful, when you go upstream, there's a long time between castles. It took us 8 hours - if memory serves, going downstream would have taken about 3. I would highly suggest going, but going the other way!
Favorite thing: If you visit Mainz in summer it might be a nice idea to go for a stroll at the Rhine River. The best way to get there from the city center is from the city hall I'd say, which is located at the "Brand", next to the Hilton hotel and the "Rheingold Halle". From there you'll already have a great view over the river too (see picture). Then walk in the direction of the Hilton hotel and after a few meters you'll see some red buildings, that's the "Kurfürstliche Schloss" which is listed as being under conservation by UNESCO. Nearby there's also the "Landtag", that's where the parliament of the state of Rheinland-Pfalz has meetings.
Favorite thing: This is the Burg Staleck which houses the youth hostel. It's a really lovely place with such a great view! Even if you're not staying at the youth hostel you've got to walk up the pathway and take in the courtyard views of the river from high up on the hill.
Favorite thing: view the many castles on the Rhine. Long ago , the owners of the castles would charge boats a toll to pass through.
Fondest memory: On this picture, taken from Mainz-Kastell, you can see the Rhine in front, the city hall on the left and the red building in the middle is the cathedral.
Favorite thing: Here's is the lonely looking street that runs along the Rhine River in Bacharach. Bacharach was a pretty empty town when I visited, maybe it was the off season?