Layovers are usually the bane of the long-haul traveler.
You're stuck with a useless block of time between hopping off one plane and getting right back onto another, with a dispiriting round of visits to the bathroom, Hudson News and the least objectionable food purveyor before squashing yourself into whatever too-small airplane seat you just escaped from an hour ago.
TL; DR: Layovers are The Worst.
Unless you're flying first-class, in Lufthansa, and connecting in Frankfurt — in which case, enjoy the Porsche.
A three-hour tour in the famous German marque doesn't come free, but hey — this promotion is only open to first-class passengers, so if you're already shelling out a few thousand bucks for a couple more inches of legroom and a lie-flat bed, another 100€ for three hours of Porsche time probably won't break the bank. Those with a bit more flex in their schedules can opt for a 24-hour rental at 303€ (included: 200 kilometers of driving; not included: gas).
As for where to head in Frankfurt, a not particularly fascinating city that was basically bombed into the ground during World War II (a historical catastrophe that accounts for the city's reliance today on skyscrapers, glass and metal)?
Avis, which is actually handling the meat-and-potatoes of the Lufthansa reservations, can answer that. They've devised four different trips, all starting at the airport and ending with "vehicle handover at the First Class Terminal" but taking in sights such as Auerbach Castle, various wineries and perhaps most importantly, some well-engineered German roads.
Just don't miss your connection.