For me, if in Japan, I'd probably bring back key chains or something to show what part of Japan I went to, but as long as it's small.

Now, although not a souvenir, I'd probably buy cds that costs too much to import by groups I like. If I ever end up going there, I'd ask about cd shops in whatever area I'm going to.

As for CD's go to Harajuku Yoyogi park on a Sunday and check out the bands playing live. Many of them offer free CD's and flyers. Not actually in the park but stay left of the park and walk along that road on the right.

Go to UT teeshirt store Harajuku where you can buy limited edition tees in plastic cans. average 1200 yen.

Loopwheeler in Harajuku for high end casual (very well made) designer JP clothing. SICK!!!!! bust out your wallet for a one off!

Go Mcdonalds and take a few menu's, everyone loves a Japanese McMenu.