I am a U.S. citizen and have a overnight layover in Frankfurt Airport. Can I leave the airport for a night out in the city and then come back? Do I need a specific travel visa?
No problem. US citizens with a valid passport can stay for up to 90 days Visa free
Frankfurt Airport has excellent public transportation connection to the city (S-Bahn). I'd recommend being back at the airport two hours or so before an international departure. It's a huge airport and lines can be long busy at times.