Music. Atmosphere. Forget tourist traps like the Floridita, even though it's a good place to start. Proceed a short walk south to the Bar Monserrate on the eponymous street, or east on Obispo to Lluvia de Oro, Cafe de Paris or two small bars on the south side of the street, the first Escabeche, the name of the second one escapes me, for cheaper drinks, better atmosphere and great music. Obispo is fun in the daytime too.
Bar Monserrate is on Avenida de Belgica, near Calle Obrapia
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Check out Willem Valloy and his band and the pizzas are great value too. You can't miss the place with Babito on the door (a mountain of a man in tux and cohiba) - he'll look after you.