Design Festa hosts the largest art festival in Asia every year, but even if you can't visit that event, you can feel the energy it holds through their gallery in Harajuku. With changing murals & art exhibitions throughout the building, it's always fun to see what young creatives in Tokyo come up with. This has been the home to many artists starting out and testing the waters for their art. Don't forget to visit the 2nd floor exhibits, as well as their other gallery building right behind the adjacent restaurant, Sakuratei (which is also a great place to eat in groups). They have a cafe / bar you can chill at with free Wi-Fi within the gallery.
3 Chome-26-8 Jingūmae, Shibuya-ku, Tōkyō-to 150-0001, Japan