Documents and other papers!

a valid passport, for children too
 

vehicle certificate
drivers license if possible an international drivers license
the allowance of the owner ( if owner is not there)
green insurancecard
valid security certificate
injection pass

On the ferry are sometimes card that need to be filled out to enter the country:

A white document "Tourist entrance" for the border police (per person 1x)
A white document "vehicle entrance" for the border police(per vehicle 1x)
A blue document"a time limited document for the vehicle" for the border (per vehicle 1x)
A white document "The taking in of goods to declare" for the border (per vehicle 1x)