What are the best ways to cool down during the brutally hot Korean summers, including indoor spots, water parks, and other cooling activities?

Korean summers are brutally hot and humid, but there are lots of ways to cool off. Indoors, big malls like Starfield Hanam, COEX, and Lotte World Mall, plus department stores and large bookstores like Kyobo, are free, heavily air-conditioned places to spend hours, and many cafes are happy for you to sit a long time. Public libraries and museums are great too. For water fun, there are big water parks like Caribbean Bay and Ocean World, indoor water parks, and much cheaper neighborhood public pools and the Han River outdoor pools that open in summer. The Han River parks themselves are popular in the evening once it cools down, with people renting mats and ordering delivery. Valleys and mountain streams just outside the cities are a classic Korean escape, and a jjimjilbang ironically has cold rooms and cold baths too. Do not underestimate a cold bowl of patbingsu (shaved ice with red bean) at a cafe as a midday reset. Carry a portable fan and water, and use the subway and underground shopping streets to move around in cool air.