A day trip from the Frankfurt Airport

Take a look at the Rhein towns of St. Goar and Bacharach. Also, consider Trier in the Mosel region. I don't believe Burg Eltz is open in March, otherwise that would make a nice daytrip.

Burg Rheinfels castle (ruins) in St Goar are scheduled to open daily from 9 AM to 6 PM on March 13, although there is a notice on the German language side of the website that it is unclear if they can open as planned due to the weather conditions this year. In any case, much of that castle, because it is a ruins, is exposed and would be difficult in bad weather. The town square is a few blocks down the hill from the Bahnhof. There are train connections from Frankfurt Bahnhof to St Goar with connections in either Mainz or Bingen, and there is a trolley from the town square to the castle.

A little farther down the river, on the opposite (right) bank, in the town of Braubach is the Marksburg, the only never-ruined castle on the Rhein. It's open from 10 AM to 5 PM after March 20, with an English language tour at noon and 4. This castle is intact and thus far better sheltered from the weather, although some of the tour is outside. Trains down that side of the river leave from Wiesbaden.

The primary boat line (K-D) doesn't start running until April.

Just went to Marksburg today. Fairly decent crowd of people there, mostly Americans. The walk up there was pretty strenuous, as the path was covered with ice, but I persevered and made it. Cool castle and I think everyone enjoyed it. I took a direct train from Frankfurt to Koblenz and switched trains there to go back down to Braubach, which was only about a 10 min. ride. Also stopped in Kaub, but got there too late to go out to the Pfalzgrafenstein. So just strolled around for a while, waiting on the next train. Oh well, just put it on my to do list for next week.

Boppard looked rather interesting and I passed quite a few other towns that I wouldn't have minded getting off the train to walk around in. It is also fun to ride the train on the opposite sides of the river coming and going, though the train going back to Wiesbaden was soooo slow.

A lot of the castles are being worked on right now, so do check with whichever one you want to see. I was quite interested in the Festung in Koblenz, but it is covered in scaffolding too.

Easy!
Try Heidelberg only an hours drive away and also connected very well by train, great castle, the old magazine/armory blew up during the 100 years war taking half a turret with it. The wall are 10 feet thick - colossal explosion.
Or for smaller, just follow the river Neckar up from there and there are four small castles on the banks on the way. Not open but they look great from the river (not sure if the river boat tour from Heidelberg is running by March 21st)
Hirschhorn is also beautiful, tiny little turret castle perched hundreds of meters above the river, a real eagles nest.
Dilsberg,also near Heidelberg an old walled town with great views.
Closer to Frankfurt is Weinheim just off the A5 autobahn
For more elegance like Versailles you can also have a look at Schwetzingen, very good example of a Rococo castle. But my favourite is Heidelberg, it's now a UNESCO protected town as a perfect example of a Baroque market town, masses of atmosphere, history and charm.
Hope this helps, Alan

Thanks you guys! I do love Heidelburg and have also done the Rhine cruise. I think we are going to rent a car and just "wing" it. Heading for Marksburg or maybe Rheinfels Ruin. I really wanted to see Eltz, but the website doesn't show that it is open until April. Jo do you know? The Mosel Valley has so many beautiful little towns as well. I guess I cann't go wrong.