What documents do I need when I fly from the U.S. to Germany? Assume I'm going for a few weeks of vacation. Obviously I need my passport. What about a "visa"? Anything else?
Germany is part of the Schengen area, so the visa requirements are the same as for any of the Schengen countries (i.e. most of the EU, excluding the UK and Ireland)
As a US passport holder, you can spend up to 90 days in any half year period in the Schengen area without need of a visa, provided you're not working. See the wikipedia page for a good introduction and overview, or have a look at the German Embassy Visa Page for more USA specific details and information.
The US dept of state website usually has all the information you need for any country. Just go here http://travel.state.gov/travel/travel_1744.html and select the country.
Here is a link to the section on entry and exit requirements for Germany: http://travel.state.gov/travel/cis_pa_tw/cis/cis_1123.html#entry_requirements
Here is what it says:
ENTRY / EXIT REQUIREMENTS: Germany is a party to the Schengen Agreement. As a U.S. citizen, you may enter Germany for up to 90 days for tourist or business purposes without a visa. Your entry into Germany begins the 90 day limit for the entire Schengen area. For further details about travel into and within Schengen countries, please see our Schengen fact sheet. Contact the German Embassy in Washington at 4645 Reservoir Road N.W., Washington, D.C. 2007, telephone (202) 298-4000, or the German consulates in Atlanta, Boston, Chicago, Houston, Los Angeles, Miami, New York, or San Francisco for the most current visa information.
If you are transiting Germany en route to other countries, make sure you know all of the entry and exit requirements for your final destination. If you don’t have the right documentation, you might be denied boarding to your connecting flight. For example, some countries (e.g., South Africa) require a certain number of blank visa pages or more than six months remaining validity on your passport.
Schengen area is the best! You get 90 days out of 180 travelling in most countries in (especially western) Europe. Notable exceptions: the UK and Ireland. More info here: http://en.wikipedia.org/wiki/Schengen_Area. Update: Switzerland is in the Schengen Area. My experiences at their border had me confused. That said, be prepared for more enthusiastic border control at the Swiss border, regardless of their Schengen status.
When I was planning my trip, I discovered that it is possible for the authorities to check you out a little bit more. Theoretically, you can be asked for proof that you have enough money to cover the trip--usually a credit card works, although bank statements may help. It may also help to have lodging booked for some or all of your stay, and a return ticket to prove that you intend to leave. All of that aside, I was never asked for any of this information, and rumor has it that Americans rarely are.
Once you're in the Schengen area, all you need is your passport to pass through to different countries, and you often don't even need that. Just be aware that if you take a bus or train across borders, authorities will often check that you have documentation. And that trips through Switzerland via Eurolines will give you a significantly more in-depth customs experience even if you aren't staying there.