I love this thread! Posting mainly so I can get some good ideas =D I will say, since I grew up doing many of these things (barefoot, hiking/camping/hunting in cold weather, pouring rain, snow, etc) I'm not sure what is voluntary discomfort vs just plain enjoyable. My childhood may have broken my perspective a bit there!
-I periodically (I try for once a quarter) go a while without alcohol. Just to make sure I can. Self-mastery and all that. I haven't done this with coffee, but probably should.
-Cold shower cycles. Similarly: swimming in lakes and rivers. I was way more badass as a kid, when I would crawdad in a glacial runoff stream in just a swimsuit. I'm a wimp now and often wear boots to do this, or stick to warmer streams. Same vein as well, scuba diving in a wet suit. Never used a dry suit, even in the winter, even for deep water dives. I don't dive anymore though =\
-Walking to the store. (Not sure this counts, I LOVE this, but I do it even in the rain)
-Living with no dishwasher, but cook all our meals from scratch. LOTS of dishes.
-Minimal climate control. I have never, in my life, had A/C where I lived. In the winter right now, we set our thermostat for 64 during the day, 60 overnight. The only exception is I turn it to 68 to shower. Wimpy, I know ;)
-Outdoor gym. We have a home gym, which we use year round. Hot in the summer. Cold in the winter (oh my god the pullup bar first thing in the morning when it's been freezing overnight... ack). Bugs. Etc.
-I suppose hunting your own meat or raising and butchering your own meat counts, but I don't do it for the voluntary discomfort aspects, but for the moral and health ones.
Hmm. Upon reflection, a lot of what I do *looks* like voluntary discomfort, but that isn't usually the motivation behind it- it's usually health, or morality, or my twisted version of fun. I'm not sure if that still counts!