Essentially, the Retiro is a large (around 350 acres), seventeenth-century French-style garden, parts of which are very formal, others more natural. Now, it is a popular place for people to pasear, stroll, especially on Sundays, when throngs of people come. There are street performers, many of whom are very good - buskers, especially Peruvians, puppet shows, jugglers, acrobats... One of Spain's most successful comedy duos, Faemino y Cansado, actually developed their stand-up skills in this outdoor environment. And you can have your fortune told, or get some instant acupuncture. 

It contains a number of interesting elements, though secondary to the experience of the paseo. The statue of El Angel Caído, the fallen angel, is the only statue to Lucifer in Spain and, it is said, the world, which may even be true. Other statues are dotted all around. The Palacio de Cristal, an enormous glasshouse, has recently been restored and is used for exhibitions. It has an attractive duck pond. The Casa de Vacas and the Palacio de Velazquez can be visited free of charge, the latter also being used for exhibitions.  The Rosaleda, rose garden, is worth going out of your way to find. Rowboats are available for hire on the ornamental lake, el Estanque.

The Madrid Book Fair, the Feria del Libro, is held here from the end of May to the beginning of June. Other open-air events - concerts, a film festival, are also held, especially in the summer.

The Retiro once contained a porcelain factory which was so successful that the French destroyed it when they occupied Madrid in the Napoleonic Wars. No traces of it remain, but you will find pieces made there in many palaces and stately homes you visit in Spain. Experts have spent years trying to recreate the porcelain made there and have recently met with some success.

Note that, while perfectly safe during the day, the park becomes dodgy as it gets dark. Even though its gates are now closed at night, it attracts vagrants, some of whom more or less live in the bushes.