If you enjoy trains, it's a very fun ride along the Neubaustrecke Köln-Rhein/Main high-speed line. The ICE does most of the route between Frankfurt and Köln at 300 km/h (186 mph) and it's a very unusual high-speed rail line because of frequent curves and grades of up to 4%.
The 3rd generation ICE trains required a multiple electric unit design with motors in the bogies of each coach to operate on the line, since traditional locomotives cannot provide enough traction and power to maintain high speeds at such grades.
Despite the curves and grades it is an extremely comfortable ride due to the superelevation of the tracks preventing you from feeling lateral g-forces and hermetic sealing of the trains keeping your ears from popping.