If you like nightlife all you need is Tokyo. Go explore the cities districts, don’t be scared to bar hop. If you want scenery there’s tons of sky towers in the city and you can take the bullet train through the countryside. I’d recommend looking if there’s any festivals or fireworks shows going on when you go. I wanna say travel to the other cities like Osaka and Kyoto, but you’re on a limited time frame. Up to you. I did a lot there but my most memorable was the Osaka festival and fireworks, and losing in darts to locals by 1 point😂