If you drink, do not drive. The attitude to driving while under the influence of alcohol varies from country to country.
Do not get involved in any way with drugs. Soft-drugs, like hasj and marihuana, are tolarated only when consumed in the so-called "coffeeshops". Possesion of the above mentioned soft-drugs is tolerated up to 5gr and for personnal use only.
Do check beforehand when hiring a car or other vehicle if there is proper insurance cover and if it covers injury to third parties.
Do travel on a return ticket and bring enough funds with you as bank transfers can take time.
Do take out medical insurance. Medical treatment is not free in the Netherlands and hospital bills (and medical repatriation) can be expensive.
Do remember that you are a guest in the country and respect Dutch Law.