Right next to Retiro Park and the Prado Museum, is the Royal Botanical Garden of Madrid. Once you pay the 3€ entrance fee, you can wander freely through the thousands of plants in this garden. At one point it belonged to the Spanish Royalty, and it was where they kept, cataloged, and studied all the plants they received as gifts or brought back from far-off places. It’s a cross between a botanical museum of rare plants, and a beautiful, calming garden. I visited it for the first time at the end of May, and it was really nice to see so many different plants I’d never seen before. If you like flowers or botany in general, definitely check it out.