Do not forget to bring hotel voucher, ferry ticket or pre-booked tickets for other events. If you have booked these via Sembo, you print them from your booking page.
Passport is mandatory for everyone in the car. The car's registration documents in original is also mandatory, as well as driver's license. Photocopies of passports, driver's license and credit card will make it easier for you if you lose them. Green insurance card shows that the vehicle has motor insurance, which you can get through your insurance company. The car's manual is also good to bring.
Do not forget standby phone number for your insurance company and SOS if an accident should occur. A European Health Insurance Card is good to have if you need emergency care in EU or Switzerland.
Reflective vest to wear if you need to leave the car on a busy road. First aid kit. Extra bulbs. The last three are compulsory in some of the countries you drive through.