You'll also occasionally be just holding it as you stand in a street car or train when no seats are available. Small and light is manageable. You create your own problems when you bring over-sized and overweight luggage. Make your trip enjoyable - don't make yourself a camel or mule. Traveler's Checks - Bring your money in this form.
Credit cards are only good for purchases in some stores.
In general, Sumitomo Banks will accept US credit cards for cash. Checks are only for banks. Upscale hotels will occasionally offer cashing services for about the same as bank rates. Click here for VISA ATMs in Japan that take foreign credit cards for cash advance locator . Dairy products other than milk and yogurt are few.