Shave ice is a big deal on O'ahu and Matsumoto is the place to go. Established in 1951 as a grocery store they now serve shave ice and sell T shirts. Crowds line up for the many tropical flavors including guava, lychee, mango and passion fruit. I recommend the guava. Matsumoto's is located in the charming beach town of Haleiwa. Spend some time exploring the town and the many small shops.