Wonderful detour from B42 between Königswinter and Linz. L331 up from Königswinter to Margaretenhöhe is smooth like a race track with several very intersting curves. If there are no cars slowing the progress, it is good fun to go up and down a few times. L253 winds down through the forest into Linz town.
In Linz, have ice cream at Ciao Ciao Gelato, next to the ferry terminal.
Königswinter is where you can show off your bike on the promenade and have your Apfelkuchen at one of the many riverfront cafes on Rheinallee.