Go and see the old coal mining... by Bagillbaer
Go and see the old coal mining areas. You'll be visiting an official World Culture Heritage Site with a particular romantically industrial charm about it. It has also been called 'the prettiest coal mine (at least above earth level) on this planet'... now, I don't know if I'd call it pretty... but it's definitely a cool place! :o) On Zeche Zollverein ('Zeche' meaning 'mine') there's sometimes exhibitions. It's quite nice, especially if you have a soft spot for gigantic, industrial architecture.
If you go there and if you're not afraid of the dark, then make sure still to be on the grounds after sunset (I think they offer guided midnight walks out there, too), it's a cool atmosphere!