If you are looking for some real warm weather then you absolutely should go to Kyushu Island. If you do so, we would like to recommend you a few of the best beaches of the island.
Just to remind you, Kyushu is the third biggest island of Japan and it is located in the south west part of the country. It is pretty easy to get here from other islands of Japan, Thailand or China. The cuisine is meant to be rich and people are famous for having warm and open personalities.
This beach is extremely popular because of its wonderful clear blue water. Moreover the sand is very light and camping is possible as well! It is located in Itoshima and you can either go there by bus or by train. If you choose to take a bus it is pretty easy; From Chikuzen-Maebaru Station take Showa Bus directly to Keya bus stop!
If you are looking for white sand then this is the beach for you! It is actually one of the longest beaches in Fukuoka Prefecture. You can either take Kubo subway line or JR Chikuhi Line.
The biggest advantage of this beach is that it is very easy to find, because it is just minutes away from the train station. Indeed, you can take JR Chikuhi Line from Mimohama Station to Chikuzen-Fukae Station. This beach is also great for a break because you can find some snacks and drinks nearby.
This one is slightly different because it is an artificial beach facing the beautiful Hakata Bay. It might not seem the most pleasant of the beaches but it is the most convenient because it is located near Fukuoka Tower and many restaurants and surf shops are located around it.
Even if you do not have time to go to Kyushu Island, you can still find several beaches all around Japan. After all, Japan is an archipelago of islands!