We highly recommend Diverbikes opposite the Retiro where you can hire all sorts of fun bikes to have a spin round the park. You can try out all sorts of models including a four wheeled bike for the whole family which hold 4 or even five and single and double karts which are great for kids. You hire the bikes by the hour at very reasonable prices. The staff are very helpful. The only inconvenience is you can't prebook so get there early on the weekends!
Avenida Menéndez Pelayo 9 - Metro O'Donnell
Tel.:91 431 34 24