No matter how good your English can be, there won't be many chances in Japan to use English ability in daily use unless you belong to a company or some kind of organization. In this case, you might join a free conversation class at English schools or online conversation sites, but it's only the first several times you can feel really excited. Speaking of me, it'll soon become mundane and I lose the passion which I had when I first started. I love to exchange opinions and ideas with many people, but speaking to the same person is not so motivating and eye-opening. I need something more challenging and more fun to keep improving English.
Then, if you're an out-going person and good at explaining things, then this will be a fun experience for you and your friends. You can register as a guide for people coming to Japan, and work as a volunteer tour guide or you can ask them to pay certain amount of reward to you.
I often wonder if I do this kind of volunteer guide for friends overseas who have the same interests to me when they come visit Japan. But there is one concern which stops me from doing it. That is, what kind of person I really am becomes apparent and I won't be enjoying at the fan sites anonymously after that.
I don't know this site is safe of not, so better read some comments from the people who have actually gone on a tour with a guide.