If you're looking for indoor activities on a rainy afternoon in El Retiro Park, consider visiting the nearby Reina Sofia Museum. This world-renowned museum is home to an impressive collection of modern and contemporary art, including masterpieces by artists such as Picasso, Dalí, and Miró. Spend your afternoon exploring the various galleries, immersing yourself in the vibrant world of modern art. The museum also offers guided tours and educational programs, providing a deeper understanding of the artworks on display.