Cycles of deliberate hyperventilation, breath retention, and immersion in very cold water. The breathing produces a strong altered state in its own right, which is the part most people do not expect. The cold produces a large and short-lived stress response that many people find clarifying.
The branded version is the Wim Hof Method: rounds of thirty to forty fast breaths, a breath hold on empty lungs, a recovery hold, then an ice bath. There is real physiology here and some genuine research, most notably a controlled endotoxin study showing an influence on the innate immune response. There is also a great deal of overclaiming stacked on top of it.
The unbranded version is older and cheaper: Finnish sauna culture, Russian banya, and winter sea swimming, all of which do most of the same thing without a certificate or a facilitator. If cost is a factor, that is the honest recommendation.
Read the risk note on this one before anything else. This is the only practice on this page with a well-documented mechanism for killing healthy young people, and it kills them in water.
A representative shape drawn from how these are typically run. Use it as something to ask against, not as any one centre's timetable.