Teaching in Japan is the best way to extend your gap year here. Japan is an expensive country and you'll find yourself rinsing your backpacker budget quicker than you can say 'harajuku'. In general, the only requirements are that you're a native speaker of English and you hold a degree. Primary schools, high schools and businesses are all open to accepting potential teachers into the fold.
In many cases, your role in Japan will not be that of a full English teacher, but rather an Assistant Language Teacher (ALT). This means that rather than being given responsibility for your own classes, you will instead be working alongside a Japanese Teacher of English (JTE). Responsibilities vary between schools, but often JTEs will be in charge of planning the lessons, and the ALT will be there to help them implement these plans and to encourage students to communicate in English.
There's quite a bit of paperwork involved in getting one of these placements, but applications open every October and it will be well worth the time investment. The most popular company to do this with is JET.
Then it's time for the waiting game until the interviews are announced for February. In April you'll find out the outcome. Your flight will be paid for by JET, with the return provided on completion of a year's contract. When you arrive you'll have a few days of orientation before heading out to your placement where the fun really begins.
You can find out more about what it's like teaching in Japan in gapper Vicki Holman's article on her teaching experience. Another gapper, Ronnie Jones reveals the ins and outs of his teaching experience in Japan here too.
Please remember that it is illegal for students on a tourist visa (not formally applying for a visa) to work in Japan. If you don't fancy teaching English in Japan which is what most of the ex pats do, then bar work is pretty much your only other option. Unfortunately, unless you're a Japanese language pro, this means you'll have to work in one of the British or Irish pubs meaning you won't meet any true locals. It can be really difficult to find work like this though – I'd recommend the first option of teaching in Japan if you want to make some real money on your gap year.