Truly the sky is the limit as to what you can see. You could spend 3 full weeks in Kyoto and still not see all that there is.
A lot of time your time should be spent in the Kansai area though since there is so much, and there are many regional passes to help you.
See:
http://www.japan-guide.com/e/e2357.htmlfor some examples.
A lot depends on what you want to see/do. Osaka is a great place for food. See www.bento.com for some ideas. Kyoto has its food too, but often more expensive.
There are many places worth visiting between your 2 end points. Some are:
Himeji (best castle in Japan),
Miyama for a tastle of old Japan:
http://www.japan-guide.com/e/e3985.htmlOkayama has its castle, Korakuen and Handayama Gardens:
https://www.youtube.com/watch?v=5zAI3_N7NEMhttps://www.youtube.com/watch?v=JGQ8o7k9GbAKotohira in Kagawa makes a great side trip and a magnificent half day hike:
https://www.youtube.com/watch?v=ley9rVIewoQKurashiki is a nice quaint place to see its old canals.
Hiroshima and Miyajima are must see places along the way.
Onomichi is a off the beaten track place with its temple trail, as well as Konsanji, a mix of a Buddhist temple and Disneyland:
https://www.youtube.com/watch?v=97Q5EjEOXI8Iwakuni with its iconic bridge and mountaintop castle are wonderful to see (and not overrun either):
https://www.youtube.com/watch?v=0COOZXi66mMThe Akiyoshido Cave and Akiyoshidai plain are magnificent and often overlooked gems in Yamaguchi.
Shimonoseki has a number of scenic sights as well, and is often ignored by the tourist books and guides.
Kitakyushu has a nice castle, Kokura, plus its old style Moji Retro area, and Sarakurayama is one of Japan's finest mountaintop views of the city (and totally empty of the tourist hordes!):
https://www.youtube.com/watch?v=Hl1pCW9IbbkFinally, Fukuoka is famous for its ramen and spiced cod roe (mentaiko), but also one of Japan's finest temples - Nanzoin - awaits you and is nearly unknown to the world:
https://www.youtube.com/watch?v=lT4VQFmHFyMThere is a good guide to the city's best sights on:
http://www.thejapanfaq.com/fukpic.htmland more previews at:
https://www.youtube.com/playlist?list=PL7LcPcmoSg-j4RLa6iC7ayvBAbOn8-0...Look into a Fukuoka Tourist City Pass while there:
http://yokanavi.com/access_tourist_eg