The old town has the most characterful bars, with a few unpretentious places on and around the main square that serve tapas and local wine to a mixed local and expatriate crowd. The Arenal has the livelier nightlife in summer, with terrace bars along the beachfront staying open late. The port area is quieter but has some good waterfront cafes for evening drinks.
The expatriate community is large — predominantly British and German — which means you will find English-speaking bars throughout town. These are competent rather than memorable; the better option is always to follow the Spanish speakers.