I love the towns along the Rhein. You can do the west side or the east side. On the east side, the most famous town is Rüdesheim which is very quaint, but also very popular. Still, in early March it won't be so crowded. It is at the outer edge of the Frankfurt area public transit network, so you can get a day pass for the trip via Wiesbaden for €16,75. Another option is to take the train via Mainz to Bacharach and Oberwesel on the west side. A € 24,10 day pass will get you as far as Bacharach, and € 5,40 covers the short hop from there to Oberwesel. These are just two highly quaint towns which are less touristy than Rüdesheim. The trip to either one takes from 60 to 80 minutes. If the weather isn't terrible, that's what I'd do.