Heidelberg is one of the "must do" for most US visitors, however, I find the city disappointing. What you can certainly do is Bonn and Cologne. Take the High Speed Train from Frankfurt. Bonn is great for German (modern) history, Cologne is great for German (Roman) history. Can be a great tour. I would first go to Cologne, spend the evening there, have a look at the Cathedral and the Roman-Germanic Museum the next morning, take the train (about 30 minutes) to Bonn, visit the Museum of German history and get an impression on the old postwar German capital. The evening you can either spend in Bonn itself (big student town) or eg take a boat ride up the Rhine. On your way back to Frankfurt avoid the high speed train but use a service via Koblenz and Mainz, taking you on one of the most scenic trips German railways have to offer - all along the banks of the Rhine.