Not Japan specifically, but I've got say I'm jealous as hell about the snack scene in east Asia.

I generally don't have a sweet tooth, and things like potato chips don't have that umami I like. I try to keep snacks around because I forget to eat, but nothing appeals to me. But man... all those pre-packaged tofu squares, various bits of marinated meat... that's my deal. There's one solid "Asian Mart" near me, I'll stock up a few months worth at a time.

Closest you get in the US is basically jerky/slim jims, which are great but expensive and kind of one note for flavor.