We drove. My family wanted to have me drive them home since they were planning on drinking, but I didn't think it was a good idea. I didn't want my first time driving on the left to be at night in a busy city with drunk passengers.I bet you could sneak a couple strong zero chu-hi's in! At least the changing area.
Or worst case, Drink all your chu-his in the green car on the way to the Onsen and then you won't need more while you're there...