A fun souvenir from Japan is a small hand towel. You can purchase them almost everywhere, in convenience stores, railway stations, and in many historical attractions.
The towels are small, light, useful, and fit easily into a suitcase. I like seeing the various images available - Jpop idols, city mascots, videogame characters, Hello Kitty from every prefecture, historical figures, and yes, the Glico Guy!
It's a unique and inexpensive item to pick up on your travels through Japan.
Guide Books on Tokyo & Japan