I brought tea! Mind you, I came after a pred who left every single teacher high and dry, so anything I brought over with me would be an improvement. Main concern was that tea bags weigh next to nothing, and for all us non-Americans weight allowance is everything.
With Tim Tams, I'd honestly ask a pred if there's somewhere in your area that stocks them. It's sneaky, but you could totally buy them once in Japan (and hey, if I can get Tim Tams - at the local Liqueur Mountain, a magical, wonderful store - where I live, most of you will at least have access to somewhere with them), and then just shop them into work in a container purchased from a 100 yen store. Less melted, more room in your bags, and it's likely that nobody will ever notice (or care).
Also, 5c coins. Just bring em in your pockets. People go nuts over foreign currency with cute animals printed on.