Radishes and such
Radishes are abundant; and one that especially comes to mind is the Daikon radish, also quite popular abroad. Daikon is a major staple of many Japanese dishes, the most prominent use being in soups, and as a topping called Daikon "oroshi".
Miso soup, Japanese winter stews, and Oden will all make heavy use of daikon for its absorbing qualities. The daikon will adapt well to any atmosphere and accept the flavor of the broth while still maintaining its shape quite decently.
Another use of daikon is called oroshi, as stated above. Oroshi is basically ground daikon radish and is topped on fish, or eaten striaght with a bit of ponzu sauce and perhaps a bit of raw egg.
The price jump in daikon between the winter and summer is amazing! Winter you can get a huge daikon for about 100 to 200 yen, but just halves during the summer will be about 250 to 300 yen (although they might be the same price in a meal at a restaurant. Get them while they're abundant!