Village Vanguard is a cool chain of trinket shops in Tokyo, as well as other areas of Japan. They sell everything from plastic models, to squirtguns, to snacks. And there is also a lot of books. One thing that is pretty popular in Japan among young people is elaborate keychain set ups.
Chain wallets are just the beginning for some. The wallet chain can connect to a belt loop which can then have many trinkets and small bags and accessories attached to climbing braces, or other novelty key chains. Even just a simple keychain attached to a belt loop is a pretty common style these days.
A cool Japanese keychain would be a great souvenir for someone back home. Try and get the style started in your country also!