How much detail should I give?Enough that someone will be able to look at your work and the declaration and say "yep, this is what he was talking about".
Should I even bother with things that don't compete with my employer?
If your contract has a typical "all your inventions are belong to us" clause, absolutely.
Is this form actually worth anything in court? (It's actually a Japanese company, I've yet to find a good resource on Japanese employment law.)
Ideally the form will prevent anything from going to court. Most people are actually quite reasonable, and the most likely way you'll end up fighting over ownership of your work is if your employer actually thinks they have a right to it.
(I am not a lawyer, have absolutely no experience with Japanese employment law, et cetera.)
