If you're staying in Frankfurt, a day trip along the Rhine River is a must. By car head west for about half an hour and you're there. If you don't have a car, consider a boat tour.
It's like stepping back into another time as you pass through old villages and castle after castle after castle nestled into well preserved terrain above the river--a whole different side of Germany compared to the city life of Frankfurt. Spend some of the time stopping in the towns along the Rhine for shopping and food. Explore a castle. Grab a bottle of dry riesling wine. Not the sweet crap you find around here--and if you don't like that stuff, don't let it dissuade you from trying a glass of dry riesling. One town worth stopping in is Rudesheim. Take the cable car ride to up above the town and over the vineyards. It gives a really great view of the area.
If you get up to Koblenz, consider switching rivers and following the roads along the Mosel River. The Mosel is less touristy and maybe even more beautiful.