What should I do during the extra 2 days I have for my Madrid trip?

Originally I was planning to visit Madrid for 3 days, but now it's been changed to 5 days.

This is what I currently have:

July 30 - Thursday:

• Arrive in Madrid in the morning
• Gran Vía
• Puerta del Sol
• Plaza Mayor

July 31 - Friday:

• Royal Palace of Madrid
• Mercado de San Miguel
• Temple of Debod

August 1 - Saturday:

• Museo Nacional del Prado
• El Retiro Park

I need to plan August 2nd and 3rd. I will fly out from Madrid on August 4th in the morning to Porto (my 2nd destination for my two city trip).

Does anyone have any suggestions on what I should for Sunday and Monday? I will be solo travelling and staying at Onefam Sungate. I only know English.

This will be my first time ever in Madrid which is why I tried to do all the touristy stuff in my original 3 day itinerary. I have no idea what to do for the next two days.