If you are staying at Grand Hyatt Tokyo, then I think this is one of great way to make use out of a concierge at the hotel. Explain to the concierge as much as you know of the person inviting you to the dinner, gender and age included and ask for suggestion for a gift from hotel concierge. Tell your budget for the gift and I am sure the concierge at the hotel will provide you with some good suggestions and where to buy gifts. The concierge can also write short note in Japanese indicating that your purchase is a gift which you can show the note to the store employee, then the store employee will wrap a gift in an appropriate way.