Solo travelling to Japan at 20 years old

Hello, Im a 20 year old guy from Australia planning on going to Japan this year for 2-4 weeks (undecided)

I’d love to go to quite a few places and experience as much as I can which includes nightlife, meeting lots of people, hiking and snowboarding. What cities would you recommend for this and what are good ways to meet people?

I’m planning on going in mid to late october until early november, would this be a good time to go weather-wise? I’d probably start my trip around the Tokyo area and finish it off up north for snowboarding. I’ll ride the bullet-train to do this.

I’m also not sure if I should book all of the hostels ahead of time, or just get the bullet train to different cities and find hostels myself.

Thanks for any tips.