Most popular place for diving is Okinawa, though you can do it in other places too, such as southern Shikoku or southern Kyushu.
The thing to remember is that most of Japan in August is insufferably hot and humid. If possible, try to plan a few days up north (I recommend Hokkaido!) to escape it. Kansai area (Kyoto, Osaka, Nara, Kobe, etc.) will be busy and hot, but it's still worth a trip if you've never been before.
To determine which cities to visit, you really ought to get an idea of what you're interested in seeing. Take a look at the Japan Guide website, buy a travel guide book, watch some documentaries. If you're wanting to go all over the place, a rail pass might work well, but if you're probably going to stay around Tokyo and Kansai, you'll be better off taking advantage of budget airlines (Peach, Jetstar..). about 5 years ago
Thank you so much. I'll buy for sure a guide. It is our first time in Japan and we will arrive at the Osaka airport. I was thinking of course to go to Kyoto, Tokyo, and Hiroshima. We'd like to stay few days at the seaside. after that I'm Still working for the trip plannig! about 5 years ago
For diving I suggest you check Yonaguni Jima, Yaeyama Islands, the site is thought to date back 5,000 years. It will offer you all the thrill you need for your trip!!! Sure planning a trip for 22 days is going to take a lot of time. You can take help of online itinerary planning tools like this one, http://www.triphobo.com/tokyo-japan, it covers many cities of Japan along with their attractions, so you can create multi city itineraries and also find places you can stay and eat.
Japan has so many interesting things to do, you will find 22 days are not enough. Check the geisha bars, edo-tokyo museum in Tokyo, Kinkaku-ji shrine in Kyoto and do include Meiji Jingu Shrine in your itinerary, it's a good way to know Japanese culture almost 5 years ago