Best nature/hiking park Japan?

In october me and my girlfriend are visiting Japan for 3 weeks.
She has a Congress she needs to attend to in Osaka the first days.

After those six days we are planning on doing some sight seeing. Here's our rough plan:

first 6 days: in and around Osaka
middle 8-9 days: train to Hokkaido to do some hiking there
last few days: in and around Tokyo

What is the best park for hiking in october? We would love to see breath taking scenery, climb some mountains (if rewarding...).

In some travelguides Aken looks very nice, but seeing photo's on flickr doesn't make it look all that nice.
Nikko (not in Hokkaido) looks nicer on flickr.

Can anybody tell us the nicest place to be in japan for beautiful nature and nice hikes?

kind regards.

If you go in early to mid October, Hokkaido, you can combine hiking with autumn colors in lower elevations. In higher elevations, such as Daisetsuzan, there is likely to be some snow, already. In late October, the chance of snow is increasing even in medium high elevations.

Especially if you go in late October, I would rather consider a more southern destination. For example, I found Kirishima National Park in southern Kyushu absolutely fabulous.

Can anybody tell us the nicest place to be in japan for beautiful nature and nice hikes?

I recommend either Hokkaido, or Nagano.

Nagano
http://www.japan-guide.com/list/e1216.html

As Uji san says South Kyushu should be good too. Gereral I fo is here.
http://www.jnto.go.jp/eng/location/interests/cherry.html

I dont recommend Nikko. It will be incredibly too crowded, especially on weekends in autumn.