Reina Sofia museum is always a good idea. And it's never enough of Picasso's Guernica as well as the other modern art gems. Save some hours for visiting and then look round the corner and have some wine at NuBel.
One of my favourite places to enjoy a beer bathed in orange lights and layered walls. They used to give concerts downstairs. Unfortunately they got recently banned due to boring local regulations - hopefully, live music will return soon amongst their palm trees and tropical lights.
Matadero is a contemporary creation center, a charming place to go at night to see the concerts and architectural lights. It always has an interesting program going on.