Art and Flamenco
See Picasso's Guernica at the Reina Sofia modern art gallery. It's free from 19.00-21.00 pm during weeknights (closed Tuesday). Saturday it is free after 14.30 pm and Sunday until 14.30 pm.
For free flamenco, visit Los Gabrieles bar (near Sol metro) on a Tuesday night, after 22.00 pm.
El Palacio Real (Royal Palace) is normally around 10€ entrance, but on Wednesday you don´t have to pay.
Check out the Prado museum, free to the public from 18.00-20.00 pm during weeknights (closed Monday), Saturday after 14.30 pm and on Sunday all day.
Museo Sorolla is also free on Sundays.
El Rastro Sunday markets and the gardens of Parque del Retiro are also perfect places for some free fun.