I recommend buying "OS-1" at any drug stores(about 200JPY/bottle). This drink can help you to avoid heat stroke. When you drink this, don't drink up in one time, please drink bit by bit.
If you cannot find a drug store nearby, buy "Pocari sweat" or "Aquarius" which you can find at any supermarkets or vending machines. Those drinks are helpful than water or any other drinks. All of them are also recommended to drink when you have a fever.
There is a candy which called "Shio-ame" in Japanese, salt flavoured candy is also good for avoiding heat problem. There are so many kinds of Shio-ame candy and you can buy at supermarkets or convenient stores and so on during summer.