head into a local grocer and go wild. all kinds of good shit waiting to be found.
definitely pick up a couple varieties of rice crackers.
beer snacks are good too (dried squid, dried sardine, wasabi nuts,etc)
i wouldnt buy any booze at the grocer though. swing buy a smaller booze shop and pick up a bottle of sake. my favorite is 'akagi-san', but it's not always easy to find in tokyo.
some good, cheap, japanese wines out there too...
if you're american - do yourself a favor and try some unpasteurized cheese... god it's awesome
depending on when your going to be in, maybe we can hang out - pm if interested