The most environmentally-friendly way to visit Keukenhof is by public transport. From March 20 till May 18, bus 858 - a direct bus - will run between the Schiphol Excellence Parking and Keukenhof. This line is also called the 'Keukenhof Express'.
This fast, non-stop service drops you right outside the main entrance to Keukenhof with a frequency of 4 to 12 buses per hour. The journey to Keukenhof takes 35 minutes.
The service is maintained by 'Arriva' so your GVB daycards are not valid here.
You can travel on the Keukenhof Express using your public transport pass (OV-chipkaart) or you can buy a ticket from the bus driver (€ 7,00 for a return ticket).
Better still, buy a COMBI-ticket. This ticket covers both the bus fee and Keukenhof entrance.