When you go to Mitsuwa, you'll be able to buy authentic Japanese food. It's often the very same stuff that you would find in grocery stores in Japan. As such, the prices are a little bit higher than I'd usually be comfortable paying, but going there once in a while isn't too bad. The selection is wonderful and it's just a fun place to shop.
I've never really bought anything from any of the other little shops that are in the same building, but enjoy browsing around. If you're a manga enthusiast, you'll probably like the bookstore, but make sure you can read Japanese because that's about all you'll find in there!