I'll be in Madrid for a quick weekend the 4-7th of April. Although I've been once, it was rather quick. Aside from the Prado, Reina Sofia and the Retiro, what should we see and do? We're into flea markets, good authentic restaurants, architecture, walking, etc.
Thanks!

El Rastro market has a great section of "old things". I think that the exact place is "Plaza General Vara del Rey"
Sunday morning until 2 p.m. approx

Yes, El Rastro is a great way to spend a Sunday morning - my advice is to go early (it is up and running from 7am) as it gets packed with people very quickly. And keep a firm hold on your bag!

It is spread over a large area between Plaza Mayor, La Latina and Puerta de Toledo - you can't miss it!

Besides that I can highly recommend the Real Jardin Botanico (2, Plaza de Murillo) - opposite Museo del Prado. We were there in April last year and the tulips were beautiful - see photo.

And beyond that, just walk and wander and see what you find. We happily spent a few days doing just that