https://www.schengenvisainfo.com/us-citizens/
This page says that
When American travelers show up at the border to enter the Schengen Zone, they will be asked by the border guard to present the following documents:
A US Passport. It must not be older than 10 years and it should be valid for at least three more months beyond its intended date of departure from the Schengen Area.
Evidence on their purpose of entry. Documents that show why the US citizen is traveling to the Schengen Area.
Proof of sufficient financial means. Documents that prove that the US traveler has the financial means to support themselves during their whole stay in Europe.
For examples of documents it points to
https://www.schengenvisainfo.com/schengen-visa-application-requirements/means-subsistence/