Recommended Day Trips from Frankfurt?

Take a train to the town of Rüdesheim on the Rhine, and then take a boat trip through the Rhine Gorge to St Goar. You'll see castles around every bend. Further north towards Koblenz is the best preserved castle on the Rhine, "Marksburg" near Braubach (do a search on this website).
Take a train to Wurzburg where you can see the "Residence" Palace and also the Marienberg Fortress.
Another option is going to Heidelberg to see that castle (what's left of it) and to explore the town.

there are some nice castles near wurzburg, in addition to the residenz and the festung marienberg. there is a ruined castle in wertheim, about 45 minutes away. and the town of miltenberg, in the same area, is worth a trip. the castle at heidelberg never fails to bring me to tears, no matter how many times i see it.

rothenburg is an easy trip, too. the whole town is preserved.

marburg on the lahn has an incredible castle on a hill overlooking the city. it has a really nice museum with everything from pottery to armor. marburg is worth a trip and possibly an overnight stay(we stay at the europischer hof. marburg has a beautiful pedestrian zone. if you go, be sure to have some wine at the wein ladl at the top of the market square. look for the picket fence. we like to sit upstairs and have wine with flamkuchen.

Here's another recommendation to see Wurzburg. Just spent three nights there with my daughter and her college roommate, who goes to medical school at the university there. A very friendly, walkable town with loads to do. And if you see the Residenz there, you can skip the one in Munich.

You can do two three interesting castles just outside of Frankfurt: Bad Homburg castle where you can still visit the private rooms of the last German emperor's wife. It looks like she'd just stepped outside. Nearby is Saalburg, and old Roman border patrol castle as that area used to be the Northern boundary of the Roman Empire. Spend the afternoon in www.hessenpark.de and if you want have dinner on the medival market square of the walled old town of Höchst down by the river Main. You're sitting just outside the castle but inside the walled town (dating back 1200 years). It's a tram ride back to downtown Frankfurt.

baden-baden or heidlberg