If the air contains many negative ions, it will give the bather a better feeling. Soapstone for example provides a good steam if it is heated properly. In a wood heated sauna, the faint scent of smoke can be pleasant, as is the scent of the vihta . To avoid unwanted smells, the sauna has to be well ventilated both in use and between uses. For the best steam, use normal, clear drinking water.
Rain water is often too flat, and salty seawater causes literally acid rain in the sauna.
Natural spring water may well be the perfect water for the sauna. As the atmosphere in the sauna tends to be moody, the lights should not obstruct this feeling. No direct lights should be used, but only bulbs behind wooden panels. The light is only needed when coming or exiting the sauna. It might be a good idea to extend the mood to the dressing room with adjustable lighting.