I've received a tip from a reader. Even though it was months ago, I'm finally sharing it with you all. Better late than never? Sorry.
This spa is definitely worth the wait, and now that I looked into it, I feel very sad to have kept it in my inbox so long. I'll let the photos tell the story.
Also, there's a foot bath with Doctor Fish, a kids land, and lap pool for real swimming. Other advertised outdoor amenities include sunbeds, cabanas, something they call "caravan camping." --->
(All photos from Spa Dogo website)
How to get there?
Well, as it appears from their website it's not so far from Cheonan-Asan, if you leave from the Onyang area. The trip could be done by car or taxi pretty painlessly. Also, there's a free shuttle bus from Cheonan-Asan KTX station that takes 35 min. There's a train stop on the cheap line called the Dogo Oncheon Station (도고 온천 역).
As another option, there's a 2-hr shuttle bus direct from Seoul some mornings (only Th, Fr, & Sa, daily during "high seasons") and back to the city in the evenings for ~15,000 won round trip.
Wikimapia of the Paradise Dogo Spa
Depending on the season, this beauty of a spa will cost you anywhere between 26,000~50,000 won for admission to the whole place. Just the sauna runs you 10-12,000. The Caravan Camping is extra :)