Depending on budget, 6hrs is too long to be wandering around in the cold unless you want to go shopping and too long to hang about in the airport, so FKK seems the obvious choice. Assuming your bags are checked through, an FKK would be like a 1st class VIP lounge with plush extras and shagging!
However getting there could be an expensive taxi. One guy posted how to get to FKK-Oase by public transport (adventurous) orvthey offer to pick you up for €100!!
I used a tantric massage joint at one point I think that is an easy walk from central station but can’t remember the name. Had a massive row with them when they switched girls. I’m normally an Eros walk-up type guy but not in this weather. Plus, if you have an onward flight it’ll be nice to arrive on the plane showered and fresh. Good luck.