People with interesting fashion tend to gather in Harajuku. (Not as it used to be though) Not sure what kind of people want to capture, but many teenagers gather in Harajuku and Shibuya.
If you want to find people in traditional costumes (Yukata) visit one of Fireworks festivals during summer. You can find many many people in Yukata and great food stalls.
Tokyo will be extremely beautiful when cherry blossom blooms so visiting there in Spring is a great idea, too.