The size of the park is immense, you could spend hours wandering through it and not see the same thing twice. There are sculptures, old buildings, waterways and of course plenty of areas to sit on the grass and chill out. Entertainers come out in their droves in the park, there is always some extremely talented performers, from singers, to dancers and classical musicians. It would be very difficult to get bored here!
For the more active of you, Retiro is also a highly popular location for jogging, playing football and even rowing! There is a lake in the middle of the park which is highly popular with both casual and competitive rowers (also and ideal way to spend a romantic evening with a significant other!)
Of course the majority of people are content with strolling through the park, taking in it's many sights and sounds. Listen to some music, watch the dancers or feed the pigeons if you are so inclined.
And let's not forget, it's also a great place to bring a few friends and chill out with a beer!