Only 7 days in Japan

I'm planning my first trip to Japan in September. I only have 7 days and want to visit Kamikochi as I love nature, outdoors and beautiful scenery. Where should I go and and stay . I would also like to experience a traditional onsen

Most people do both Kyoto and Tokyo for the first trip to Japan. For 7 days 6 nights (7D6N), something like
- Kyoto(3N) - Tokyo(3N) -.

In you want to add Kamikochi to the above, that would be
- Kyoto(2N) - Kamikochi(2N) - Tokyo(2N) -.

You might consider dropping either Kyoto or Tokyo; say,
- Kamikochi(4N) - Kyoto or Tokyo(2N) -

If so, you could have more chance to explore the region; Kamikochi(1D), Shin-Hotaka(1D), Norikura(1D), and Takayama(1D).
https://www.env.go.jp/park/common/data/17_chubu_map_e.pdf
https://www.env.go.jp/en/nature/nps/park/chubu/guide/view.html

There are many Onsens in this area. Nakanoyu Onsen would be the nearest to Kamikochi, but Hirayu Onsen offers better connections to the other sites. Shirahone Onsen is good to Norikura. Shin-Hotaka Onsen for Shin-Hotaka Ropeway. Good reputations come from Fukuji Onsen in Okuhida Valley.

Here are some pickups based on Google Reviews.

Fukuji/Okuhida Onsen:
http://www.soene.com/
http://www.kakurean.com/en/
http://www.cyouza.com/
http://www.okuhida-gh.com/en/

Shin-Hotaka Onsen:
http://www.yarimikan.com/english.html
http://www.karukaya.co.jp/en-gb/
http://shinzansou.com/english/
http://okuhida-sangetsu.jp/
http://shinhodaka-yamanohotel.jp/en/
http://www.okuhida.or.jp/en/roten_catalog#shinhodaka2

Hirayu Onsen:
http://hirayunomori.co.jp/contents/english/
http://www.okadaryokan.com/eng/
http://okuhida-sanganoyu.jp/en/

Shirahone Onsen:
http://www.shirahone.net/english.html
http://www.awanoyu-ryokan.com/awanoyu/english.html
http://shintaku-ryokan.jp/
http://www.shirafune.net/
http://www.shirahone.org/english/

Nakanoyu Onsen:
http://www.nakanoyu-onsen.jp/

Hi! September can be very hot though it depends on the year. Kyoto in particular can be nasty in September, so if days over 30 celsius arent your cup of tea, then maybe consider cooler places.

Kamikouchi is very nice, but you want to be there when the buses are gone, so stay overnight. Early morning in particular is great, with the monkeys getting about. But be careful on your walks - not because of bears (which I didnt see, though we saw a decent sized snake there), but because of the monkey crap which is pretty much everywhere, and if you step in it it is a mission to clean it off. I'll never eat pesto again...

We stayed at the Onsen Hotel. It is brilliant, and the staff are lovely. The food has an emphasis on natural produce, and the rotenburo is awesome. If you are lucky you might get monkeys in the trees near the baths. We stayed 3 nights which was great, though 2 would be plenty, even overnight at a pinch. You could always go on to Takayama which has some nice sites and the park is great for hiking. Also the squirrel park just out of town is awesome, if you like squirrels (and what kind of a person would you be if you didnt?).

Another nice onsen town is Bessho Onsen. It has good water and some decent temples. It is about an hour out of Tokyo. There isnt much to do there, so overnight is plenty. The ryokans are really cheap, too, with great food. We stayed at Uematsuya. Bear in mind horse sashimi is a specialty of the area, but you can always say no.

Our favorite place is Nara - we like it much more than Kyoto. It is green with massive parklands and some forests and there are deer running about. 3 days there is enough to recharge the batteries. We usually stay at the Nara Hotel - old wing. Check it out!