[tazzy]

You don't need a teaching degree to be a teacher in Japan for a while.
Wait, scratch that, to be a teacher in Japan you need a properly accredited Japanese teaching degree (i.e. very hard for you to be a 'proper' teacher here).
To be an assistant teacher or somesuch in Japan, you don't need a teaching degree. I know lots of people in such jobs and they did a vartiety of courses- graphic design, physics, history, etc...
And some of these gigs (e.g. the Jet programme, you should look into that) pay just as well as new teaching jobs- others of course pay quite awful, I've a friend who works for a company called interac and...yeah, he isn't happy.

And no, business most certainly does not make you more money in the long run. Studying 'business' doesn't mean you'll be a big businessman. It is a valid degree, but doesn't give you any special leg up over people who did other stuff. In fact, sorry to say but in my uni it had a bit of a reputation as a drinking degree.

All depends on what you want to do in the future. Do you want to be a teacher? Then totally yeah study teaching, it won't hurt your chances of coming to Japan for a few years. Will boost your chances of getting a job perhaps (if not one as an outright teacher).
Or do you want a job for a company doing....whatever it is people who work in normal offices do? Then study whatever interests you, if this matches up with what business courses teach then great, study that, some people do it for genuine reasons. All too many kids though think studying business is the route into business whilst in actual fact 90% of degrees do much the same thing only they give you a more interesting 3/4 years of uni in the meantime.
My best friend studied computers because he thought it would be more useful in the future than his real interests in politics and history- he had a miserable time at uni, graduated with low grades and has had a right pain getting a job...other friends who studied less immediately practical subjects like literature and the like though, and managed to get good grades due to enjoying their subject, have done pretty OK.

And I'd greatly recommend against taking a gap year unless you really do have something awesome to do in that year which will really help your future career/you really need to work and earn money for school/your parents are mega rich and can get you a job at their company whenever you want.