Take a swimming pool and cover the bottom with Strontium-90 in the form of something insoluble, maybe strontium titanate. Not only do you have free and reliable heat source, but the emission of beta particles into the water will cause Cerenkov radiation, which will give the pool a healthy blue glow that will look awesome.

From a safety standpoint, you could put some webbing down to provide some separation between the users of the pool and the strontium, but beta radiation won't penetrate very far into water anyway.