As its the beginning of a new year, all shops are selling 福待(lucky bag). Its $500yen for $1000yen value stuff. Thus you will be able to notice people camping overnight at certain stores in megamalls like Sunshine City and Tokyo Hands, especially Hello Kitty store and computer devices store. Few from my tour group bought lots of Hello Kitty stuff as they claimed its really cheap there but they spent around $2000yen or more haha!
air ticket

sunshine city

