I'm thinking of very nice experiences I have had in Grimbergen and Boetfort in Belgium. These spas had heated indoor and outdoor pools, hot tubs, steam rooms, and saunas of various temperatures and interiors. Not to mention places to lay down and relax.
I've been to a spa in Pardubice that had all of these amenities, but I haven't heard of anything in Prague. It would be nice to find one in the range of about the 1000 CZK per person for a couple of hours. Within an hour's drive of Prague would be the farthest I would be willing to travel.