Mt Aso is quite interesting and can be combined with a visit to an onsen, there are plenty nearby. But this is best done by rental car from, say, Kumamoto.

Mt. Koya and staying at one of the temples there is another possibility.

In late August you could go to Mt Fuji 6th station or even, if you so desire, climb Mt Fuji. It may be just another mountain but it's not at all like any mountain I've seen in the Canadian Rockies.

August is also a good time to visit Hokkaido.