Clothing

Black Montbell Plasma 1000 jacket: most optimized winter jacket for lightweight travel, comparing warmth to weight. Very compact. However, pricey and with limited lifespan (3-7 yrs) due to lightweight materials being less robust
Montbell rain jacket: Japan knows about rain. If I want a rain jacket, may as well get one of theirs. They have many designs/features to choose from. Unclear if they use materials that cause cancer :)
Montbell U.L. lightweight windbreaker: super lightweight and compact wind breaker. They have an even lighter one, but it looks like you’re wearing a brightly colored trash bag, so I went for this slightly heavier but more fashionable model (heavier by maybe 10-20 grams - optimizooooor mode activated)
Montbell hiking pants (the tighter fitting model, not the looser one): good for rock climbing, hiking, and general activewear long pants
2 pairs montbell socks: if you’re interested, may as well try these out. I just bought them for fun.
Montbell climbing socks: if you’re interested, may as well try these out. I just bought them for fun.
Montbell attachable pouch bag: I wanted one of these shoulder pouch bags and what better way to show off “I went to Japan” than by having a Montbell branded one. I conveniently found it at a used clothing store for cheaper
Uniqlo underwear: literally the best underwear for lightweight travel that you can find. And cheap compared to alternatives, like $9/pair. Buy one to try it out, then if you like them, go back and buy a lot more to get over the minimum to get the tax-free discount https://snarkynomad.com/the-best-mens-underwear-for-every-guy-out-there/
Uniqlo running sleeves: good for hiking and running as an alternative to sunscreen or just extra warmth (it’s like converting short sleeve shirts to long sleeves)
Uniqlo merino gear: honeslty probably good to just wnader around uniqlo, they’ve got some good clothing for decent prices. Wish we had a branch in Eastern Europe, I think only London, Paris, and similar have Uniqlo
Shokz Openrun Mini bluetooth running earbuds: I wasn’t planning to buy these but I like them for running https://shokz.com/products/openrun
You should research computer gear and electronics shopping in advance because you can go WILD with this while you’re there. You can literally spend 1-2 hrs in Yodobashi Camera just admiring their audio gear, not to mention their massive keyboard collection, etc. and it’s all on display available to test out. And I haven’t even mentioned Akihabara in Tokyo or DenDen Town in Osaka. It’s just insane.

Tea gear

Ippodo matcha sieve: good for breaking up the matcha particles, useful for the most premium of matcha experiences
Kyusu teapot: these are special for Japanese sencha. You could buy the one that Ippodo sells with their branding, but if you want to save some cash, you can search for ceramic stores or tea stores and buy a cheaper one there
Gyokuro hohin: different than a Chinese gaiwan, but similar, and mostly for gyokuro
Matcha bowl(s): Buy this in Uji, the capital of matcha, if you want a bowl with a cool story behind it. Basically just pick ones with a design or color you like. Also consider whether you want a tall bowl that is easier to whisk without droplets flying over the rim of the bowl
Matcha whisk(s) and spoon(s) and whisk stand(s): get as many of these as you want. I think I saw matcha whisks for sale somewhere in Belgrade, so no need to buy too many. They last for 6-12 months depending how often you use them, but they are disposable (which is why they aren’t displayed in the tea ceremony show-and-tell, because they’re heirloom quality items like the other tea ceremony utensils)
Consider visiting antique stores if you want to buy an old school natsume (lacquerware storage container for matcha). I didn’t buy one buy they are cool. You can also find unique teapots or teacups in these stores. This is the one I went to in Uji and bought that super fancy teacup
Sign up for tea experiences at the tea museum in Uji (Chazuna museum) and you’ll get a free ceramic thingie for wagashi (the sweet that comes with matcha). The ceramic thingie is made by handicapped people, so you can feel good about getting free stuff
You could consider a tour of a tea farm (like obubu in Wazuka) or a tea processing facility (like this one). There is a tiny free tour showing the tea processing steps at the Fukujuen HQ

Tea

Ippodo premium gyokuro and premium sencha: don’t look at prices, just buy the absolute best that Ippodo has. If you are at the Kyoto store, the top tier tea actually ISN’T ON THE MENU - they have a secret premium tier for the more discerning tea connoisseurs
Matcha from Kanbayashi in Uji: This is the 16th century store that has the emperor grade matcha. Go upstairs in the store to see their museum display showing all their newspaper clippings and other history of delivering tea to the emperor. Ideally the Belgian son-in-law is working when you’re there (40 year old white guy), he’s fluent in English (and Japanese, naturally) and can help with any questions
Genmaicha: green tea with rice. One of the cheapest green tees, but I like the taste and its good for casual daily enjoyment without breaking the bank
If you’re in Kyoto, I can recommend buying tea from Mandaracha and Garden Lab. They have unique hipster offerings, like single origin teas and rare tea breeds