There's probably not much snow (if any) in September, so I dont know if the snow monkeys will show up - I think they wander further afield when the weather allows. I have never actually seen any in the wild, but then again nearer tokyo a lot of the larger wildlife has become quite rare.
The jigokudani monkey park is actually right in the middle of 'mountain country' so from there you have loads of hiking opportunities. You may want to purchase a "bear bell" to keep the bears away while you are walking. I personally find the noise quite irritating and I guess the bears do too, but its definitely much better than getting eaten.
BTW, some things which spring to mind assuming this is your first visit...
Japan is more cash orientated than the UK - a suprising number of businesses wont accept plastic. ATMs usually charge a fee which increases outside of office hours. Convenience stores which have ATMs are very common in the cities and generally seem to open 24hours, though in small villages you may well be out of luck.
Eating out in japan is much cheaper than the UK - though touristy areas do tend to get inflated prices
Buses and trains are quite good value in the cities, slightly less so out in the countryside, but taxis are pretty expensive everywhere.
Gratuities (tips) are not the custom in Japan.
Talking of customs, you may end up visiting a temple while in kyoto. Just like entering someone house you should take of your shoes before entering. Keeping a hat on seems ok though. Usually there is a gate into the inner compound which has a raised sill. You should step over this not on it (I dont actually know why this is though, other than to protect the wood work?)
Edit:
Hiring a car is quite practical and familiar if you drive in the UK, and many roadsigns are bilingual japanese/english. Driving may be the only way to access some hiking trailheads. Parking is a problem (i.e. expensive) in cities but usually quite ok (often free) in the mountains. Petrol is reasonable if you are used to UK prices. Speed limits (in Km/h) are quite low which makes for relaxing driving when traffic is light. One thing to watch out for is that a green (blue) light for go often (though not always) means go for pedestrians to cross the side road i.e. watch out for crossing pedestrians when turning at any traffic light controlled junction.