There is a nice walking tour of Frankfurt done by native English speakers. It was way better than riding a bus around. It is called Frankfurt on Foot and they are associated with Insider Tours in Berlin, www.insidertour.com  This is a fun, 3 hour walk around the city and you get to see a lot of the major sites with enjoyable commentary too. The price is right, as it is only 12 euro with a discount for seniors, students and military. They walk every day, rain or shine, at 10:30 leaving from the Römer. Look them up under www.insidertours.com. You get to visit the Dom, the Holocaust Memorial Wall, learn about the Stumble Stones, go in the Alt Nikolai church, see the Old Opera house and lots more. In June, July and August, they will have a 2nd tour in the afternoon, called "Where Once Was a Wall" which will walk around the park where the city wall used to be. This tour starts at 15:00. You can find out more information here: Frankfurt on Foot