If there is a chance of sunshine, especially in the dead of winter please take advantage of the sun and go bike riding in Parque del Buen Retiro. By Bike has affordable rentals €4 per hour or a tad cheaper if you increase your usage. If you are into roller blading the costs are the same for rentals. The bikes aren’t anything special, mainly just cruiser bikes so they can get uncomfortable after a long haul but definitely worth it for a 1 or 2 hour spin through Retiro. It is always nice to lay out in the sun along the lake but nothing beats a good bike ride through the park. It really is the best way to explore every bit the park has to offer in the shortest amount of time.
Check out the website for rental times and prices as they vary from season to season. But in general they are cheaper than most bike shops in Madrid and located right across the street from the Ibiza entrance.