Visas are not needed by citizens of the U.S., Canada, Ireland, Australia, New Zealand, or the U.K. for visits of less than 3 months. You do need a valid passport, unless you're a citizen of the E.U. (in which case you need only an identity card, though we recommend you always carry a passport anyway).
For specifics on what you can bring back into the United States, consult the U.S. Customs & Border Protection (CBP) website.
Unless you're arriving from an area known to be suffering from an epidemic (particularly cholera or yellow fever), inoculations or vaccinations are not required for entry into Germany.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.