Japanese securities companies have special accounts which automatically report tax-related information to the tax agency and withdraw taxes appropriately. So you'd be able to do standard stock and currency trading without being required to file a return. It's quite nice being able to work at a company and participate in the market, and still not have to worry about filing.

Doing rarer activities such as investing in startups brings you into return-filing territory though, but I guess that's unavoidable since it's private investing, and not a concern for most people. Reading many comments on this post, it's clear hackers generally aren't "most people" ;)