Muji may be my favorite store in Japan. As a college student, its as if someone had peeked into my brain, anticipated everything I ever needed on an average day, and put it all into one store. Everything is inexpensive, minimalist, and functional. For instance, Muji has a whole range of traveling equipment, from back packs to tooth brush kits to about 40 different sizes of miniature shampoo bottles. It has socks, slippers, a complete clothing line (more expensive than Uniqlo, but still reasonable) that sticks to conservative greys and browns. If you forgot anything on the plane or back home, chances are you can buy it here.
Smaller Muji "kiosks" abound in train stations and shopping malls. These usually stick to a few food items, stationary, and clothing. The larger stores are the ones to go to. The Yurakucho Muji is particularly large, and even has a clothing alteration service and a deli/restaurant.
Tote bags are pretty useful here, and for 100 yen you can purchase your own plain white Muji bag and customize it with pens and rubber stamps at the register. I've found these make fun, unique souvenirs.
Muji has a house brand of tea, snacks, etc. that are quite inexpensive. In addition, Muji also carries hearty freeze-dried soups and meals. Oh, and Muji carries bicycles and furniture too.