Do you know if they like wine? IMO candy would be safer (and a 3000 yen bottle of wine in Tokyo is nothing), although imported single malt scotch is a classic business gift. I would avoid flowers or something that could possibly have cultural symbolism of which you aren't aware.

Whatever you get should be wrappped beautifully. It's also good for gifts to have famous brands or prestigious store labels. [Let the store warp it for you in their box or with paper/ribbon having their logo.]

If your host and hostess have spent a lot of time abroad, in some ways their expectations and ideas of etiquette for you on this occassion are likely to be closer to those of Americans living in Japan.

In any event, it's truly a special honor to be invited to dinner in a Japanese home. [As I'm sure you know, be prepared to remove your shoes at the door, so your shoes and socks should be in good condition.]