Minimum layover time required: 6 hours
My European readers have complained (justly!) that I have yet to write any posts about the continent, which got me thinking about this wonderful 8 layover that I had in Frankfurt.
Follow signs for the S-bahn, Frankfurt’s commuter rail that 
gets you to the central train station, Frankfurt Hauptbanhof, in less than 15 minutes. From Haupbanhof, take tram 11, direction Fechenheim Schießhütten-straße 4 stops to Römer / Paulskirche. (A Frankfurt transport map can be viewed here. )
Your Frankfurt adventure begins at Paulskirche, or St. Paul’s Church, an example of early Lutheran church design and the birthplace of German democracy.
Next, take a short walk 
down to Römerberg, the “medieval” city hall square that is arguably one of Frankfurt’s most noted landmarks. (Medieval is in quotes because while the square remains true to its original architectural style, the 1405 square was destroyed in WWII and subsequently rebuilt.) Weed through the throngs of tourists to catch a glimpse of the fountain of justice, and note the plaque commemorating the Nazi book burning .
Then, make your way down towards the waterfron
t. Those with the time may wish to pop into the Historisches Museum Frankfurt (which sadly was being renovated when I visited).
Afterwards, a relaxing option — assuming that jet lag is beginning to set in after a red eye flight — is to hop onto one of the Main river cruises which launch from the Eisener Steg bridge. Commentary will focus on the different architectural styles along the river banks — interesting but certainly no match to Chicago’s river architecture cruise. The main draw is soaking up the sun and appreciating the blue sky
After your “snooze cruise” it’s probably time for lunch. (Naturally, lunch can be enjoyed at any point along the way depending on your schedule!) Make your way past St. Leonhardskirche to Hauptwache where skyscrapers abound. Schillerstrasse pedestrian street takes you by the Frankfurt Börse – the hub of the city’s finance district. From the Börse, Finally, meander along the Zeil, Frankfurt’s main shopping / pedestrian street, where shops and eateries abound.
Allow yourself at LEAST 2.5 hours to get back to the airport before your scheduled departure. From Hauptwache, catch the S-bahn (S-8 or S-9 direction Wiesbaden) back to Frankfurt Airport.
And there you have it — a lovely layover in Frankfurt!
PS: Should your layover fall over a weekend, another option is is try The Ebbelwei-Express Historic Tram Tour, a hop-on hop-off tour on Saturdays, Sundays and public holidays. Get your bearings with a downloadable podcast, enjoy a glass of apple wine (Ebbelwei) and even try your hand at learning basic Hessian. (Full disclosure — I haven’t had the opportunity to try this but it certainly sounds fun!)