Most Polish people will head to mountains of southern Poland for their spa experiences. There are several popular spa towns in the south, but I don't know how practical only a 1-day trip will be. In these towns usually the spas are associated with hotels, they give a holistic health resort experience with spa/food/activities etc. This has been traditional getaway for local Poles since communist times (think the beginning scenes in the movie Grand Budapest Hotel). But now these spa hotels have been renovated post-communism and are very nice.
The most well know spa town is Zakopane, with easy connection to Krakow by bus or train. Personally I go to the Silesian Beskids for the spa treatment, they are a mountain range a little further from Krakow, Wisła and Ustroń are the main spa towns in this area. I always try to go to Hotel Belweder when in the area, I believe they have a location in Zakopane too - https://hotelbelweder.pl/en/