Yeah, I guess I have to agree with Deadite in the end. HARD-OFF is generally too out of the way to visit if you're on a short stay in Japan. Hell, I've been living in Tokyo over a year now, and I still haven't bothered getting out to the Kanto HARD-OFFs yet. It would use up too much of a day, and I might buy too much stuff to carry back to the nearest train station too.
Also, though you can make great finds, like Deadite said, HARD-OFF is all about luck what you find. So, could end up being a total waste of time too. (In my case, I still enjoy looking at all the old stereo equipment, PCs, musical instruments, laserdiscs even if there's no retrogames.)
Sorry to pretty much retract my whole previous post. Stick to the big retro shops in Akihabara and Denden. It's the best use of your time.... and spend the rest of your time seeing historical Japan!
Edit: There's also a HARD-OFF in Akihabara, but they don't have games there. If you can read Japanese, you can check the listings in the link I posted above to see which locations have the "game" logo.