Flights from and to Copenhagen
As København is not that far away, I thought about taking the train there. A quick search with Deutsche Bahn showed that a single trip would cost around 160 EUR and would take around ten hours. A reasonable surplus for being environmental friendly is worth paying for me, but there is a point where this surplus becomes outrageous. Then I researched flight connections from close by airports and the only affordable direct flights from Köln-Bonn airport are actually the cheapest ones operated by Ryanair.