If you are an american citizen, what do you need besides a passport to travel europe?

I’m planning on flying into Britain and then I want to travel to the European Union. Is travel between britain and the EU fairly simple? I’m finding the best priced tickets into London, but I don’t want to stay in England, so I just want to know if its no problem to just cross over into France and travel the rest of Europe.

What in the non peak season to travel in Europe?

We are planning for our Europe trip. From the tips that we read, reservation for trains are required and strongly recommended if you are travelling via Eurorail during peak season. There’s also this peak season surcharge that we want to avoid. So when exactly is the non peak season to travel Europe? We had London, Parie, Swiss, Italy and Germany in mind.