Located by the river and near the new Museu de Leiria (and part of the admission price), the Moinho do Papel was Portugal's first paper mill to be moved by water (1439). These days, it's an educative hands-on experience and illustrates how hydraulic energy was used to grind grain, plus there's an olive-oil press and paper-making workshop. A separate room outlines the history of paper (English descriptions) and the mill.
It hardly makes the 'don't miss' list, but can be fun, especially for kids.