If you're even just slightly romantic you should, when you're around, certainly visit the Drachenfels.
Okay, it's only a mountain if you're a lowlander like me (it reaches 321 metres), but even the Germans have decided it is part of the Seven Mountains (Siebengebirge) and on the day we went up that mountain we walked right into the clouds (which were of course hanging very low that day ...).
More importantly the Drachenfels in 1836 became the first natural reserve in the world, when the Prussian government bought the quarry on the mountain and ended its activities.
Official protection had to wait until 1922, but by then two poets had made the Drachenfels immortal: Lord Byron and Heinrich Heine. Lord Byron mentioned it in his Childe Harold's Pilgrimage, Heinrich Heine made the mountain the subject of his Die Nacht auf dem Drachenfels.
And then there was Richard Wagner ... One of the legends surrounding the Drachenfels is that the dragon Fafnir lived in a cave on that hill and was killed there by Siegfried, who, bathing in its blood, became invulnerable (apart from one detail, that is). That was the reason the hill was called "Dragon's Rock" and also why in 1913 a Halle referring to that legend, the Nibelungenlied, was built on the hill. In that Nibelungenhalle you'll find paintings and statues referring to Richard Wagner's operas and a ... reptilarium.