Isetan is Tokyo's most fashion-forward department store. Head to the 2nd-floor Tokyo Closet and 3rd-floor Re-Style boutiques in the main building, and the 2nd floor of the men's building to discover new Japanese brands that haven't (yet) hit the big time. Other reasons to visit: the homewares from contemporary artisans (5th floor) and the excellent depachika (basement gourmet food hall).
Bonus: there's a rooftop garden where you can sit and eat all the goodies you've amassed in the food hall.