Obviously, you need a passport. Beyond that, check with your local travel agent to see if your home country has a visa treaty with Japan. If you're a US citizen, for example, you can enter Japan for up to 90 days without a pre-arranged visa (they stamp your passport upon entry). Most of the online advice I've seen also advises having your return ticket and enough money to support yourself for the duration of the trip but I've never been asked for either. YMMV -- a long-haired hippy with a backpack probably would be asked to prove he can support himself while he's in Japan ;-)...