Txirimiri

If you're visiting Madrid, you can't miss La Latina on Sunday. La Latina is the bourough where young people goes to have some Tapas on Sunday.
It's very close to el Rastro, a kind of market on the street where you can find almost everything, so you can take a walk in the morning by El rastro and go have some lunch at La Latina.
You'll find a lot of bars and restaurants in this bourough, and plenty of people. To have somo good food you have to go to Txirimiri. More sophisticated Tapas than the typical ones, you may try the rissoto among the big variety of delicious tapas they have.
You can eat in the bar or try to find a table in the back. The restaurant is small and gets crowed, so it will be hard to get a table. You'll beter try the bar ther, and it's worth it!

Not too expensive and with good amience.