The weather during tsuyu is unstable. It is important to prepare for rain at anytime. Although it's typically rainy with lower temperatures, sometimes it has less rain and can be hot. The front often brings heavy rain in Kyushu region. If you are visiting western regions, you need to be more aware of the probability of rain.
The main problem during the rainy season is the humidity. People tend to become irritated by it. Taking a bath or shower often helps you keep comfortable. Also, things are easily covered by mold. If you are staying in Japan, it is important to air your closets or houses when the sun is out. As many cases of food poisoning occur during this season, it's also important to watch out what you eat and not to leave food out of the refrigerator.
Although tsuyu is a gloomy time, the rain is very important for rice cultivation. Also, there are many pretty flowers blooming at this time. One of them is ajisai (hydrangea) which is a symbol of the rainy season in Japan. A variety of irises also bloom during this season. You can view them in many gardens and parks. It might be a good idea to hike small mountains near you or take a walk to neighborhood parks. Viewing pretty flowers makes us feel relaxed on gloomy days.