I would suggest Canada, in the Rocky Mountains (Burgess Shale and Drumheller would be neat places to visit as well, both within a few hours of somewhere like Banff), but maybe (probably!) I'm prejudiced because that's where I live. It would be good for Americans (distance wise), and better for some Europeans, due to the exchange rate, and there are tons of things to do every season.
Otherwise, I would agree with Copenhagen or Frankfurt.
Otherwise, I would agree with Copenhagen or Frankfurt.