Namak

Hello. My friend and I are planning a whirlwind 2-weekish long "backpacking" trip (i.e. we will be utilizing stylish backpacks and staying in hostels) through Europe in December. Because we are about 1 week into our plans, there are no plans other than the completely unrealistic ones we've made so far. So, can anyone recommend a part of Western-Europe that we should absolutely visit/avoid? Right now our tentative plans are Hamburg-Amsterdam-Paris-etc.-etc.-etc-back to Hamburg. We're wide-eyed girls from a small town in deep, dark Africa, so anything beyond dirt and sand would be exciting. One more thing, can someone explain what "10 days within 22 days" means when it comes to Interrail Global passes. Does that mean that if our trip is 22 days long, we can hop on and off trains as many times as we like, but on only 10 of the 22 days? Thanks a lot.