[Gabcom]

When I went to Japan with school (not whole school, only 20-30 people), we were suggested to give gifts unique to Australia, or just something Australian that they most likely won't find common in Japan. So I (my mum) bought all these random things like little glass figurines of Australian animals, toy boomerangs for the kids, koala purses for the girls and some other random stuff that I wouldn't really want.
Frankly, I think any kind of gift is fine, as long as it looks like you put a little thought in it (all that counts). If I assume that they think the same as me, the gifts they give you will most likely end up in some corner of a room.
Can't agree with giving them a bottle of sake though, it's something they can just go down the road (or wherever) to buy, doesn't really go well with the experience of hosting a foreigner kinda thing if you know what I mean. That, and they'd probably be giving you a bottle of sake to take back to the family (I received 2 bottles from 3 different families).
And thats what I think.