I officially got bored of regular baths n showers so here's a variation of both:
Make the bath water as hot as possibly bearable and fill it so with you inside it's right at the brim. Get in. wait until you've got your body temperature up and become uncomfortably hot and try and get your whole body in the water. Then while still in the hot bath, unplug the drain and turn the shower on on MAX COLD. Hide in the hot water and as it drains out your body temperature slowly cools and you find yourself sitting in a freezing shower still sweating... I've been doing this the last couple days it's a bit like a makeshift turkish bath effect... heating and cooling your body rapidly.
Another thing I've been doing is making a really hot bath, and making a cold bath with two bags of ice thrown in.... Go from bathtub a to bathtub b changing your body temperature really fast.... this requires two baths and honestly I don't do this when I'm home alone in case I accidentally put myself into some kind of shock and drown lol. But after you're done you feel like a million bucks!
Have you been to a Banya or turkish bath? The one near me uses a giant cold water barrel and there's always ice inside.... so you go into the hottest sauna of your life and then when you can't take it you go into the ice barrel or they even have what's called the "cold surprise" which is a lever you pull and a giant amount of ice water hits you from above. It gives you head aches the first few times but after that I found myself chilling in the ice water as if it was a hot tub. Afterwards I didn't need to wear a coat outside in -15C, it and it felt great! this is the one I used 2 years ago on my bday they exist it's a thing
edit: I saw old russian ladies getting into ice water... so I agree it's probably dangerous but i assume you get acclimatized to it