Good point that I didn't think of. The Japanese relatives will almost certainly have gifts for the visitors. Visitors should bring something in return, preferably nicely wrapped. Even if not wrapped to meet US security requirements, bring something from home as a gift. From my home region it might be some local produce like almonds or raisins or wine or perhaps a picture book and/or tee shirts from nearby Yosemite. A local connection makes it more personal and appreciated.