South of Waimea and northwest of Hilo, there is some amazing backcountry snowboarding off the summit of Mauna Kea (13,800 feet). There is an access road that is plowed a few days after each snow storm. So you can drive or hitch to the summit and ride down. There can be snow year round, but the most reliable is December through March. May can have 6 foot dumps though!
The road to the summit is located near milepost 29 of Saddle Road. The drive to the summit requires 4WD.