I've always been fascinated by ukiyo-e woodblock prints and this is the best place to see them. Tucked away on a quieter street than most in Harajuku, it's a calm oasis in the middle of chaos. Exhibits change monthly. The exhibit I was most fortunate to see a while there was a portion of Hiroshige's "One Hundred Famous Views of Edo." It was absolutely stunning. It's a small space with limited pieces on display at a time, but each piece is so detailed that I stayed for about 1.5 hours. Just a wonderfully curated museum.
The gift shop has a large collection of Japanese tea towels that make good souvenirs.