The Casa de la Música occupies the broad outdoor staircase descending from the Plaza Mayor, and on most evenings from around 9pm it becomes one of Cuba's most enjoyable live music experiences. A band sets up at the bottom of the steps, the audience settles on the stone stairs above, and the music — typically salsa, son, bolero, or a mix — fills the warm air. You can buy rum and beer from the bar and sit for hours.
It is touristy, unambiguously: the bands know what visitors come for and they deliver it. But the music is genuinely good, the setting is genuinely beautiful, and the mix of tourists and locals (many of whom dance) makes for a lively atmosphere. Come after dinner, expect to stay late, and don't bring more cash than you need. The steps fill up fast on weekends; arrive early to find a good spot.