Area between Shinjuku and Shibuya near Yoyogi Park. This is Tokyo's fashion district. The area's fame started to grow during the Tokyo 1964 Olympic Games. This area has become a a fashionable place. Here you'll find the famous "Harajuku Girls", Shopping, luxury brands and cosplay.
原宿 is fun and a must visit - but also quite young and crowded. I'd make a visit there for the crepes alone (you'll find many crepes stalls along Takeshita dori) or Laforet which is a great department store for fashion. Otherwise you'll find many small and trendy shops on the opposite side of Takeshita dori, behind Omotesando Hills.