3 days in Frankfurt

A friend and I are going to spend 3 days in Frankfurt at the end of July. What are some must sees and must dos in the city of Frankfurt. We also need a train to Prague on the 30th. What is the best/cheapest company or pass to do this? Thanks

There is not a whole lot to do in Frankfurt, however, when I was there last year we took a day trip to Heidelburg. It was a lot of fun. I would defiantely recommend going to Heidelburg for a day while your in Frankfurt.