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You can buy a lot of electronics in Japan that you can't get elsewhere. Japanese companies typically test products out on the domestic market first before releasing them to the export markets (That's why my phone is cooler than yours.)
If you're a music fan a lot of artists release Japan only versions of albums. (This may be to make up for the sky-high prices.) Check your favorite band's discography and see whether they have any Japan releases.
Japan is not a very cheap place, so don't expect many bargains. You're more likely to find things that are unavailable elsewhere than cheap stuff.