At first I thought "I need a passport". That part is true for sure. US Citizens need a passport to get BACK to the US from just about anywhere, so that's important.
Whether you need a visa or other documentation is country specific. Most of the countries in the EU (Germany among them) are part of the Schengen Agreement. The Schengen agreement allows easier travel between these countries (i.e. less paperwork). It currently includes the US in it's reduced travel requirements.
Whether you need a visa or not can (thankfully) be found on the internet. I found a website that let's you pick your destination and purpose and bam, out comes whether you need a visa: https://visacentral.com/
Ultimately the Schengen agreement says: if you're from an approved list of countries (which includes those in the agreement naturally) you don't need a visa. Since the US is approved, you don't need a visa.
YOU DO NEED A PASSPORT though. Your passport is your travel document, and, conveniently, it's the only one you need.
Note: For some parts of Europe, you are also obligated to carry extra medical and travel insurance. Germany does not have this requirement, but if you plan on traveling around Europe, that's something to check on.