Agreed, it is more fun to travel with someone you know and like, but it is not a prerequisite for having a good time. And sometimes it can be an advantage. For example, if you have a specific interest or hobby which your travel companion doesn't, you don't have to negotiate where you go and what you do. So, think of your interests and hobbies, and what you'd like to see, and just set yourself the objective of doing exactly that.
If you like baseball, think of going to a game in Japan, put your mind to the task, finding out about where to get tickets, game times, how to get to the stadium, and then just go. My general attitude to travel is that even if you end up hating the place, it is always interesting.
If you like trains, just head to Tokyo station and get on a Shinkansen out west. Pick a major station an hour's ride away or so and just get on the train and ride it. Enjoy the ride. When you get to your destination, just buy a ticket back. That should take you a big chunk of the day, and the experience of hurtling along at 200 miles per hour is pretty memorable.