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 round-trip ticket).
Better still, buy a COMBI-ticket. This ticket covers both the bus fare for a round-trip ticket plus the entrance to the Keukenhof. You can buy COMBI-tickets online via the link on this page, at the Holland Tourist Information boot at Schiphol or at the small van parked next to bus 858 at the bus stop.
You can also use your Amsterdam & Region Day Ticket! This will cover your bus fare only so you will have to buy an entrance ticket for the Keukenhof separately.