Of course, the most important thing to bring with you is water. Keep a couple bottles with you in your bookbag and make sure to drink throughout the day to avoid heat stroke, and heat exhaustion. You can easily buy these at the local connivence store, or even at the various vending machines that you find around Japan. If you wish to cut down on plastic waste you can also use a large reusable water bottle.
2. Sports Drinks
Besides water, you should also carry some sports drinks with you as well. When you sweat you're not only losing water, but also essential electrolytes, salts etc. You can also suffer from water intoxication if you drink too much water throughout the day, so keep some variety of sport drinks handy. You can buy these at connivence stores and vending machines as well.
3. POW! Instant Ice Pack!
Regular ice packs eventually melt and lose their cool, and you have to keep it in the freezer for hours before they're ready. Japan sells ice packs that are filled with gel that will immediately freeze when you punch it, and they stay cool for hours. Wrap these in a towel and put it around your neck to stay cool during hikes or other outdoor activities.
4. Deodorants and Wipes
While roll on deodorants don't really keep you cool, they do prevent you from smelling bad. I've mentioned in a previous article that Japanese deodorants aren't as effective as western deodorants, so I would advise to bring your own. However, you can also buy deodorant spray that can give you a fresh scent as well as a cooling mist. I would suggest dove or degree spray if you choose to bring some, it sprays on dry and don't have an overpowering scent.
Japan also sells wipes, or sheets. These come in a pack and can be used to cool you off while leaving a pleasant scent. These sheets come in a variety of scents, and can be found in the deodorant section of the local connivence store.
5. Fans: New and Old!
Fans are a traditional way to keep cool during the summer and can make a great fashion accessory! You can carry around a traditional hand fan to keep you cool during the summer, or if your arms are getting tired you can buy a small portable electric pocket fan. Some electric pocket fans have a mist feature so you can spray yourself while the blades run.
There is a variation of the hand fan called Mizu Uchiwa or water fan. These fans don't close, and can be dipped in water to add a cool breeze. These fans, in my opinion, are far superior to the electric pocket fan as they don't run out of batteries!
6. Portable Shade!
You may see some people carrying around umbrellas, this might seem a bit strange, but it's nice to have some portable shade with you. If carrying around an umbrella all day seems a bit overkill you can wear a visor, or even a large sunhat to keep your face from the sun's rays.
A nice combo would be to wear a sunhat with a wet towel around your neck. This will keep you relatively cool for a few hours, but be sure to re-wet the towel every so often so it doesn't dry out.
Hopefully, this helped you figure out what to bring with you when you visit Japan in the summer. The most important thing is to take your time, and know your limits, especially if you're doing a lot of walking. Don't be afraid to take a minute to sit down, and rehydrate, or grab a bite to eat. Happy travels!