Tourismus+Congress GmbH Frankfurt am Main organises walking tours around the city. A two hour Historical tour, with an english-speaking guide would set you back 95Euros.
Want to ride? Ride the ebbelwoi express. It's that old red tram that goes around the city. You can sit back, eat a brezel and drink the local ebbelwoi or apfelwein (apple cider) while you look around. Best for the lazy ones! ;-) An hour ride costs 5Euro, which includes the snack and the drink.
Have money to spare? Take a 30-minute helicopter ride and discover the city from the sky. This option will set you back 185Euros.
Discover the city from the Main and Rhine rivers. Primus offer tours that vary from 45 minutes to a whole day's tour. Tours can be booked at the Eisener Steg (that bridge, near the Römer).
If you'd rather save and walk, you can do that, too. Head off to the RMV kiosk across the Hauptwache for that FREE map of the city, which also highlights the best places for tourists. From there, you can freely walk around and discover the city.