Autoexperiments

I consider it the greatest failure of my education that nobody ever taught me to take, organize, and archive notes of any kind. I've only just begun that life-long project in the last few months. That said, I'll share what thoughts I can on each of the experiments you've specified. Maybe I can at least point a finger in the right direction for you.

Cold Showers

This is the one I tried the longest ago, and had the least observable results. In the end I quit because my only noticable result was that I ended up hating the idea of getting in the shower. I recall being left with the impression that it would probably take more cold exposure than showers, and over a time period of some years to produce the promised biological changes anyway. YMMV

A quick look on Google shows that the current claims about the benefits of cold showers are far more varied than I remember. When I tried it, there was some talk about brown fat production as a result of repeated exposure to cold temperatures leading to more aggressive metabolism and better cold tolerance. As I recall, that conversation converged on a much stronger genetic component than environmental. Now I'm seeing talk of blood flow, pain relief, immunological benefits and more. To me, that smacks of snake oil, but provided you avoid hypothermia I don't think you'd be doing yourself any obvious harm to try. Be sure ahead of time what benefits you want to see and how you intend to measure them, of course.

Sugar

My understanding is that we've known for several decades that sugar is the enemy of good health and dentistry, so there should be plenty of literature out there. At some point the sugar industry made successful efforts to blame our problems on fat instead, eventually kicking off the low-fat/fat-free craze that still seems to have a stranglehold on popular American diet advice. I recall some reporting early in 2016 (I think) about the influence the food lobby has on our dietary guidelines. By memory, the current recommendation allows about twice as much added sugar than is actually thought to be "safe".

I've tried for some time now to reduce my sugar intake, and succeeded only somewhat. It turns out added sugars are extremely difficult to avoid (around here, at least). Nonetheless, my doctor seems pleased with the way my routine bloodwork has been slowly trending over the last few years. Still, given the addictive nature of sugar and the sort of cumulative damage it apparently does to us in the long term, I doubt a couple of weeks will make a noticable change. The cravings may not even disappear for longer than that. There are plenty of articles to be found talking about how unpleasant sugar withdrawal can be.

Gratitude Practice

Here, at least, I feel I can point you in a good direction for more research. Last couple of years I've been keeping a regular meditation practice and absorbing a ton of theory on a variety of related topics, including gratitude practice. You can search by topic on DharmaSeed.org to find many experienced teachers' thoughts on gratitude practices. From there, and from "Mastering the Core Teachings of the Buddha" by Daniel Ingram [Amazon link] I've learned that, while the changes this kind of practice makes to the mind can begin to be evident in a couple of weeks with intensely focused practice, it is likely to take years of regular practice to solidify those changes. Unless you plan to start with a 2-week gratitude retreat, I wouldn't expect half a month to be enough time to notice the effects of the adjustments you're planning to reprogram your brain.

All this isn't meant to discourage you, but rather to encourage patience. I definitely wish you success with all your experiments, and I'm sure we'd be interested here in seeing a follow-up for each when you have your data.