Make sure you get your shots.....the world is a dirty place.
Research your destinations, and check your timing...you dont want to go to venice in the rainy season when it's underwater, or france in the month when half it's people go on holiday and the shops are closed, ect. ect.
A stopover in Europe can be combined with a rail pass to see multiple countries. I suggest reading a lot of Frommer's guides. Learn about local customs before you go, japanese feel a toothy smile is a sign of aggression, and an O.K. symbol to them is like flipping the middle finger to us. Taking time to learn the cultural differences before you go will make for less awkward moments.
Sounds like a wonderful adventure, good luck.