Go to the basement of a department store and select something in a box that costs between 1000 and 3000 yen, like senbei or manjū; if they don’t like it, at the very least they can regift it, so they will be pleased to have it. In general, wine or other alcohol is a fine gift for men, especially professors, but it can be an inappropriate gift for people you do not know well. Go with something nicely wrapped, don’t spend more than five minutes of your life picking it out, and don’t spend less than 1000 yen (you’ll look cheap) or more than about 3000 yen (you’ll look desperate to please).