Best Way to get from Farnkfurt Airport to Downtown Tourist Sites

Frankfurt Flughafen (airport) is on the main train line that also goes to Frankfurt Hauptbahnhof (Main Station). It takes about 12 minutes. Check Bahn.com for the day you need the information.

this might be useful (starts from Terminal 1) - https://www.youtube.com/watch?v=U5t7t1glSe4

The rail station is connected to the airport.

I stayed at the Hilton and I had a good time watching the passengers either running from the airport to the rail station.

It is quite fast, with a train taking about 13 min. (4 stops from the airport) to the Main Train Station – Hauptbahnhof, or 6 stops to the Hauptwache which is the closest stop to the Römer and Altstadt. There is usually a train every 15 min. and they run from very early in the morning until quite late at night.
Use this website to find times, though it isn't really needed. www.rmv.de

Simply follow the signs to the Regional Train Station.

If you land at Terminal 2, simply take one of the free shuttle buses, or the Sky Train, which is a monorail that takes you to Terminal 1. It should take about 10-15 min. to go from Terminal 2 to the Regional train station.

Cost – a single ticket, one way (einzel Fahrt) costs 5.00 €. This ticket will take you into Frankfurt city center by train, and is also valid to get to your final destination by tram, bus, S-Bahn or U-Bahn, as long as you keep traveling without a break. An all-day ticket (Ganz Tag) for one person that includes travel to and from the airport as well as all public transportation in the city, is 9.75 €.

I'll just note that of the German cities with a major international airport, Frankfurt is by far the easiest to visit during a layover. As the others have mentioned, the train ride into town is very short and uncomplicated. Enjoy!