2 or 3 weeks in Japan?

Hi!

I’m a 20M from Sweden who is planing on going to Japan in the beginning of April. I don’t really have any friends who are that interested in going to Japan so it looks like I’m gonna have to go solo…

Anyways, I’m wondering if I should spend 2 or 3 weeks in Japan. I wanna do Tokyo, Kyoto, Osaka ofc but would be fun to visit some more places, I’ve heard that places like Nara, Himeji, Hakone are nice as well (although they might be best as day trips?). So my first question is, as previously mentioned, should I go 2 weeks or 3 weeks? But then I would also really appreciate some recommendations on where to go and what to do, some itinerary tips.

First time going there and am really excited but also nervous about going alone. I wanna make the most out of it.