Visiting towns and countryside around Frankfurt

I will be traveling through Frankfurt on my way to India and back to the States. I have some free time and would love to visit the countryside or towns but not sure what i should do. Looking for advise on how to spend my time.

Head south on a train from the Main Station (you can get a train from the airport to the Main Station) to Heidelberg. The train parallels the Bergstrasse and has some of the most beautiful countryside in Germany. You will get a better view if you sit on the side that faces the mountains (on the left in the direction the train travels). If you will have a car with a GPS, plug in Weinheim, Rippenweier, Oberflockenbach (all in Baden-Wuerttenberg, but just an hour away from Frankfurt and very accessible). Stop in Heidelberg if you can and walk up the Philosopher's Way for a spectacular view of the castle and Neckar River.
Another option is to get to Heidelberg and take a short boat trip from Heidelberg to Neckarsteinach (about 2 hours or less of a trip). Beautiful countryside and at least 4 castles to see. If you don't want to head south, suggest driving east for about 45 minutes to Buedingen (not so inspired countryside, but cute town), north to Schmitten in the Taunus (better countryside and some mountains) or north to Ruedesheim and Assmanshausen (Rhein River area). In Ruedesheim, you can take a half hour gondola ride across the top of the vineyards overlooking the river (spectacular). My first choice, though, would be the Bergstrasse and Heidelberg. Have fun!!!

Marburg