Everything u north will be quite cold but my personal favourites for hiking (wich i did this summer) were:
Rebun Iland :wich is over Wakkanai in Hokkaido and most definetly to cold by now,
Japan 2008/2009: Rebun Iland 2
Hakkoda-san: wich is an hour under Aomori and might still be do able. Book a night in the Sukayu onsen hotel and they will pick you up in Aomori. Let them drop you off on the Ropeway and take it to the top. Than its a nice 4 hours to the Onsen. enjoy the Onsen. BUT, this 4 hours involve climbing up to 1500meter, so check about trackcondition before you get there, i hiked in Oktober wich was still alright.
Japan 2008/2009: Hakkouda-san, Sukayu Onsen
Muroran: there is a hike (well better call it a walk) form the youth hostel to the cape in Muroran (under Sapporo) wich is quite nice and takes about an hour.
Japan 2008/2009: Muroran
Towadako: There is also a nice hike wich goes along the river (i forgot the name). This one should still be doable
Japan 2008/2009: Towada ko (Towada See)
I added some links though you probably cant read german, you can at least watch the pictures of the hikes. I havent traveled anything south of Tokyo yet, so thats all i have for now. Im sure there are lost of nice tracks, but i just wrote about the ones i did.