Today, many tourists visit the famous ONSEN places all over Japan. If you don't like to go to too popular ONSEN places, I would like to recommend "SARUGAKE-KYO" Onsen in Gunma prefecture, where lesser tourists yet visit, compared to those popular places.
There are several nice spots on autumn leave view, which can be recommended near "SARUGAKE-KYO" Onsen.
1) The first place is near the Lake "AKATANI". You can walk around the lake on the natural trails and take a good panorama view from high viewing point nearby.
2) The second place is near "Komagata Valley" in the small river "AKATANI'. In autumn, you can enjoy a beautiful scenery with very coloful leaves, which is surrounded by quiet river and highlands. You can see the red bridge over the river. This is where I really like to recommend to everyone. Just for your info, this bridge is also popular for some peoople who love to take a high "Bungee Jump" from there, where the bridge is 62m high.
3) The third recommended place is the Ridge of "MIKUNI", where you can take an entire view for Lake "AKATANI" and surrounding forests from the highland. If you can drive to the Ridge of "MIKUNI", you may stop a car and take a great photo shoot.