The rules vary by country and you should double check current requirements before setting off. If you are a member of a motoring organisation, it should be able to provide up to date information, as should the national tourist board web sites, or see here. At the time of writing, the most stringent requirements required for all of France, Netherlands, Germany and Belgium are:
carry your passport, driving licence, vehicle registration documents and certificate of insurance
display a country of origin sticker (GB etc) on the back of the car
use mobile phones only with "hands free" kit
children should sit in the back
all passengers should wear seat belts
fit beam converters to the head lights of a right hand drive vehicle
- carry a warning triangle and spare light bulbs
It is mandatory in France (and spreading to other countries) to carry a reflective vest, fire extinguisher and first aid kit.
Both foreign registered cars and hire cars are targets for thieves so never leave anything, not even a jacket or jumper, visible in the car when you leave it. If you have to leave your car unattended with bikes on it, eg for an overnight stop en route, you would do well to find a secure car park.
Fines are levied on the spot and must be paid in cash (Euros).
For further details, click here.