1988

 1988.

 Below, I am the 4th from the right, and I am not even participating a little bit.

 I'm not sure when Grandpa went to Japan, but here are some pictures.  It must have been around 1987 or 1986 because I do remember him bringing me back a little mechanical dog.

 These pictures below are not dated either.  I don't know what month Grandpa's birthday was in, but I'm going to guess that these were taken also around 1988 on his birthday or Father's Day.