Must have:

Driver's license and passport

Green card - order from the insurance company

Roadside assistance form

Environmental Obolat - Germany

Claim form

Reflective vest for everyone in the car (Recommended in Germany - everyone who gets out of the car must wear a vest in case of an accident)

Warning triangle

Europe

Nice to have:

Bicycle rack - cover + warning plate (EU requirement and mandatory in Italy/Spain)

Parking disc - Germany has its own unique disc. The ones used in Norway and Denmark are not approved and you can get a fine.