I just got back from vacation in Germany, and while I was doing research for the trip I found a spa near one of the places I was staying that does radon therapy. From what I could gather on the web site, you sit for multiple sessions in a room with walls made of crushed radon-producing rock, reading, chatting with your neighbors, or having a snooze. Also, they ensure the radon levels comply with the German Spas Association's guidelines and you need a doctor's prescription to get treated.