Europe may not seem like such a big continent, but there are so many countries and cities that you cannot see on only one trip. I suggest either going in depth into a certain region (i.e. Scandanavia, Eastern Europe, Iberian Peninsula, etc.) or doing a trip through the heart of Europe but visiting only the biggest cities. I have given you a list of popular countries in Europe. Click on the link and it will show you information on the country and interesting cities worth visiting.