Have multiple cards you can pull cash from in case the local machines are finniky about one or the other. Also, I tend to use my debit over there since the fees arent as high as my regular credit cards. That said, its nice to have those extra cards around when the debit card doesnt work in a given machine.
I will second traveling light. And have a suitcase with the little rollerblade style wheels.
Wear the most comfy walking shoes you possibly can get. Many areas of Europe have cobblestone streets, and it can be murder on your feet after a few days of sightseeing.
Use a passport wallet and keep it securely attached to your person. And have a few photocopies, just in case.
I would brush up on my French if I was going to Paris. The French seem less inclined to English than in Amsterdam, where its fairly common.