Greece may not be renowned for snowboarding or for its winter sports in general, but there are some great locations for snowboarding and they won't be jammed. The costs are very low compared to the "big" destinations elsewhere. Lift tickets run 15-20 Euro a day, and snowboard rentals run about the same. Many of the lifts are tow lines, but most resorts have some chair lifts as well. The relatively low prices won't last forever, so enjoy snowboarding in Greece now.
This resort in the Peloponnese provides a snowboard park and a snowtubing park. One of the more popular skiing areas, the prices and waits can be higher here. On New Year's Eve, they throw a party in the parking lot. The "Ski bus" from Athens serves Kalavrita. The site's English version is still light on English; here's help translating it
or any other page in Greece.
This site is mainly in Greek, but some notations are in English. Board Generation operates a bus to Parnassos and Kalavrita on Saturdays in the snow season. Expect to pay 12-15 Euro. Their telephone number in Athens is 210-671-3879; add 011-30 if you're calling from the States.
Sadly, this hasn't been updated in a long time, but it gives a brief look at the start of Snowboarding in Greece.
Find out more about the options for snowboarding in Greece from the World Snowboard Guide.