Have to echo zkzkz's advice on getting out of Western Europe--esp. if you're interested in lifestyles/history/architecture that are different than the US.

I'd devote say 5-7 days for a taste of the North and East, then move on to the great cities of Central Europe.

Start with the open jaw into, say, Bergen, and then work your way to Stockholm for the cruise ferry (cheap and beautiful, and a very neat cultural experience) to Tallinn. Am a bit biased in favor of Estonia but it's a good way to get the idea of the Nordic and post-Soviet cultures/histories at the same time, and easy enough to then move on to the great cities of Central Europe.

Happy to talk about the region more by PM.