There's even a drug store in DonQui!
Japanese health products and supplements are really popular! A lot of Japanese products focus on anti-aging, anti-oxidants and immune support, so people even in the U.S. order them online. At DonQui you can buy them tax-free and at discounted prices.
Lunch boxes (obento), and thermoses are also very popular with visitors to Japan. They are made well in Japan because they are used so often by Japanese, and they make great souvenirs for to use at home or great gifts.
Cooking ware, especially ceramic knives, are also really popular. If you're planning on bringing these back, though, check with your airline about their regulations and restrictions.
DonQui is also a great place to stock up on all of the interesting Japanese KitKat flavors and other snacks!
There is also a ton of Japanese makeup here! The prices are great, too, so if this is something you like, DonQui can be a great place to stock up.
For visitors with tourist visas, your purchases will be tax-free, so make sure to visit the Tax Free counter!