[JohnBraden]

Don't give four gifts, even if there are four in the family you want to gift. Four and nine are considered unlucky. But since you didn't want to leave the father out, then nevermind!

Practical gifts are better than decorative. Perhaps a nice set of glass coasters? Or perhaps snacks from home that you know aren't available there. It's good to give something to be enjoyed by everyone but then not had to be stored or put away, since space may be at a premium. Nuts or dried fruits? I don't know how much Japanese people are into cheese, but maybe a cheese "set" or whatever it's called?