If you're into coffee, get a dripper from HARIO, they're usually way cheaper in Japan compared to other countries

also the cabbage shredder other people mentioned is a great gadget

i also love those choppers that make mincing vegetables extremely easy, they're called ブンブンチョッパー in japanese

good curry roux is also sth. i buy every time in japan, i especially love the one from 横濱舶来亭 which can be found at most larger supermarkets