Not sure how much use I can be, but i feel bad that no one else has bothered to answer yet!
I gave my "host" family (it wasn't arranged, I didn't live with them, they just kind of adopted me anyway
) some stuff from the UK that had been made in my hometown; traditional sweets and the like. Not sure how easy that would be for you, but my advice is;
- make sure it's nicely wrapped, even if it's cheap,
- offer something perishable like food, or things that will eventually be used up like cosmetics and candles, as they may not want or have room for decorative items.
- If you can go for something with personal meaning or wit rather than flair and expense. The best received gift I ever gave anyone? A jar of homemade greengage jam. Try and think what sort of things you've done with them or what they like.
Maybe you could confer with some other japanese friends? Say you want to give an oseibo (an end of year thank-you gift, which is basically your intention), but you're not sure how to go about it.