Take exit 16 (Nieuwegein / Papendorp) from the A12, then follow the signs to P+R Papendorp. Every seven minutes you can take a free bus from here to the city centre. The nearby bus (Nr. 24, 29, 195 and 229) will take you to the Central Station in exactly 20 minutes. And back ofcourse... At the weekend take minibus Nr. 229.