An interesting and recommended walk will let you experience Harajuku and Shibuya, and all of the trendy places in between.
Starting from the Takeshita exit of JR Harajuku station, walk straight away from the station down Takeshita-dori, where you will almost certainly run into the mingling pedestrian crowds. When you reach the first major crossroad, Meiji-dori, turn right. When you reach Omote-sand?, turn left. Here you will enjoy the tree-lined streets and pass by stores with some of the latest in fashion, and perhaps, as this author experienced, pass by a large open-air banquet area where a loudspeaker declares someone man and wife, and lets the whole world know about it.
When you reach Aoyama-Dori, turn right, and you will pass United Nations University on your right, and Aoyama Gakuin University on your left, before continuing down and finishing up at Shibuya's world-famous pedestrian crossing.
Allow yourself approximately two hours for this walk.