Present in 30 countries, Interrail is a train pass allowing you to make your own trip regarding of course your expectations. Travelling by train is probably one of the best way to travel. You enjoy the landscapes without having to stay focused on the road and it's the kind of place where you can meet a lot of interesting peoples. From the Scandinavian countries till Turkey, passing by Poland or Western Europe, you can almost travel everywhere in Europe.
So now you just have to choose what kind of trip you want to do. You want to discover the best beers in Europe? (of course drink with moderation) In this case start your trip in Belgium, go then in the Netherlands, continue in Germany and finish in the Czech Republic. You want to go back in time and see the Hellenic and Roman heritage? Go to the Acropolis in Athens, follow the Adriatic coast till Dubrovnik, Split and the amphitheater of Pula in Croatia to finish in Rome. You're more a museum person? No problem! Go to Madrid to see the Prado and Reina Sofia museums, stop then in Bilbao to see the Guggenheim museum and continue your travel in the direction of the north to see the Louvre and the Pompidou Center in Paris and finish your trip in Amsterdam at the Rijksmuseum.
Of course, those are just suggestions among the many things you can see in Europe. (I haven't even talked about the train in the mountains that you can find in Switzerland or the one in the heart of the Norwegian fjord)
The thing with the Interrail is that you can really do the trip of your dreams. There is a lot of different offers according to your needs. You want to just travel in one country? Fine! There is an Interrail pass for you. You want to travel during a month? Perfect, there is also an Interrail pass for you.
Now, no more talking, go plan your trip!