Frankfurt has excellent public transport. The S-Bahn takes you from the Central Station north to the Opera, then via Fressgasse to Zeil and Konstablerwache, before turning south across the Main to Sachsenhausen.
The U-Bahn has two major lines useful for visitors: one from the Central Station north to Berger Strasse via Theaterplatz, Römerberg and Konstablerwache, and another going around the centre into Sachsenhausen. You can change at Konstablerwache and Theaterplatz.
Frankfurt is also bicycle-friendly, with many bike lanes and polite drivers. The best place to rent a bike is at the Central Station; otherwise try Radschlag in Hallgartenstrasse 56.