Eurotrip

In September my Uni friends and I are hoping to travel Europe via rail for three weeks. Next week we are going to plan a route and book our train ticket and a few hostels. So far on our list we have Amsterdam, Germany (bit vague as its a huge country..) and Croatia. I've been to a few Greek Islands, Cyprus and Spain but other than that I've not seen much of Europe, just beachy tourist resorts. Anyone recommendations would be appreciated!