When traveling to Europe, you will need a passport. Most European countries, including Great Britain, France, Italy, Switzerland, Belgium, Spain, and Germany, are parties to the Schengen agreement, which means that Americans can stay in the country for up to 90 days with a passport. Beyond 90 days, you may need a visa.
Pack as light as you can, and research the local climate before doing so. For those planning to play in the Alps, some warm clothing, such as a fleece jacket and hat, and a rain/snow jacket of some sort, is an absolute necessity. Many travelers to Europe like to bring everything in a backpack of some sort; a pack that holds 3,000-4,000 cubic inches is a good thing, and a half pack can also double as a hiking backpack. If you are planning on doing more sightseeing, low-topped hiking shoes can double as your day shoes. However, for more formal sightseeing, bring dress clothes and dress shoes.
European electric systems are different than those in the United States. In Europe, the voltage is 220; in the U.S, it is 110. Many newer appliances, including many camera chargers, are dual voltage capable, so you don't need to bring a voltage adapter. Check your appliance for its voltage capability.
Regardless of whether your appliance is dual voltage or not, you will need an adapter plug to be able to plug into the outlets in Europe; the adapters are different for Great Britain and Ireland versus on the continent. Many electronics stores sell universal plug adapters. Bring one of each.