What are transportation options to visit Keukenhof in April?
You can buy a combination ticket from the Keukenhof website when they go on sale that includes bus transportation from either Amsterdam airport and Amsterdam.
You can also take public buses that will drop you off near Keukenhof. You can use Google maps from wherever your hotel is and look that up.
According to the website, tickets go on sale in mid-November, so you won't have to wait long for definitive answers. When we were in Haarlem, we got a combo ticket for the local bus and garden admission. From Rotterdam, we took the train to Leiden, where we changed to the "tulip bus". You can't miss the bus, it's spring green with lots of tulips painted on it!
Donna