Nanki, a generic name for the southwestern part of Wakayama, has an array of various amusement parks, zoos, beaches and places where you can enjoy water sports. Approximately two and a half hours away from Osaka by limited express train, Shirahama is a great place for a weekend trip not to be missed.
https://en.japantravel.com/wakayama/senjojiki-a-thousand-tatami-mats/27392