If you have the time, I'd take the train from Frankfurt to Rudesheim am Rhein. This charming little town is a UNESCO world heritage site. Its only a little over an hour by train from Frankfurt. Spend the day exploring the town a bit, take a cruise on the Rhine to see the ancient castles along the Rhine, get off the boat and explore another charming town, St. Goar and it's castle, Rheinfels Castle. Take the train back from St. Goar to Frankfurt via Mainz.
And don't forget to buy some of Rudesheim's tasty potent Ausbach candy for your colleagues, friends and family! The chocolate-covered cherries are my favorite. (You can also buy them in the duty free shop at Frankfurt airport.)