After a day of sightseeing, you might like to relax in the Plaza Mayor. You can sit at an outdoor cafe and have a drink or two. There are people of all ages there, and you'll probably see some buskers and street performers doing an act. Everything from musicians to fire juggling/poi.
Dress Code: Anything goes.
Plaza Santa Ana is in the heart of the tourist area where everyone goes to eat and drink and hang out in the terraces.
At night you often have street artists on the square and when it's really warm out, its nicer to be here than in a bar with no air-con.
Having said that, prices are higher here and it can get very very crowded during summer weekends.
The Delic is located in the Plaza de La Paja, a charming little square in the central neighborhood of La Latina, one of my favourites.
The summer terrace is a lovely place to spend the afternoon or evening, as well as the inside bar which by the way serves a pretty decent mojito. The tapas are also excellent...
The crowd tends to be local and trendy.
Dress Code: Come as you are!
This centrally located square has many nice bars and terraces (Cerveceria Alemana, Miau...) and is a perfect place to sit on the summer nights, to have something to eat or drink...