There is one thing to understand before buying anything: in Japan you wash before you get in the bath. The tub is only for soaking. Every item below makes sense once that is true, and most of them make no sense at all if it is not.
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If you buy two things
Bath salts and the thermometer. Together they cost under forty dollars and they change the experience more than anything else here, because most disappointing baths abroad are simply too hot and too short.
Can I do this without a Japanese bathroom?
Mostly. The salts, hinoki, thermometer, cover, pillow and charcoal all work in any tub. The stool and bowl need a bathroom where the floor is expected to get wet — in a carpeted or drywall bathroom, shower first instead and skip that set.
Will bath salts damage my tub or plumbing?
Carbonate salts are fine in acrylic and enamel tubs and fine in normal drains. Check before using any of them in a jetted or whirlpool tub — most manufacturers advise against additives, because residue collects in the jet lines. Sulphur-based onsen powders can also mark stainless and some coatings.
What is the rule that changes everything?
Wash completely, outside the tub, before getting in. Soap, scrub, rinse — then soak in clean water. The bath is not for cleaning; it is for heat. This is why a Japanese family can share one tub of water in an evening, and why an unheated foreign bathtub full of soapy water is a different thing entirely.
How hot and how long?
40 to 41C for 10 to 15 minutes. Hotter than 42 raises your heart rate rather than relaxing you, and long very hot baths are a genuine health risk for older people — this is a recognised seasonal problem in Japan in winter, when a cold changing room and a very hot bath are combined.
Do you really reuse the water?
Yes. Everyone washes first, so the water stays clean, and the tub is covered and often reheated between people. Many Japanese washing machines can also pump the leftover bath water into the wash. It is a water-saving habit as much as a cultural one.
Is hinoki worth it over ordinary bath salts?
It does something different. The salts change the water; the hinoki changes the room. If you want the smell of a Japanese bath, nothing else gets close, and a bag of offcuts costs almost nothing compared with the wood itself.
When should I not take a hot bath?
Straight after drinking, straight after a heavy meal, or if you have a heart condition without checking with a doctor first. Warm the changing room in winter rather than stepping from cold air into 42C water — the sudden temperature change is the actual risk, not the bath.