It's erm... either a back massager or a vibrator. Nothing really epitomises kitsch and the whole cult of kawaii (cute) better than the bubble-headed cat Hello Kitty. What could be a more appropriate souvenir of subverting this wholesome image? Just pray you don't get stopped at customs!
Kabukicho in Shinjuku and Shibuya have many ...ahem... shops for adult entertainment which are almost all staffed by sweet old ladies or young students making a yen or two. Hello Kitty products are easy enough to spot.
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