Antigua is a small colonial town, not a party city: the night gets going around nine, most bars stop serving about one, and you can walk the entire scene in ten minutes.
Cafe No Se
Candlelit mezcal bar on 1a Avenida Sur, deliberately dim and shabby, with live music most nights and its own mezcal poured from a back room through a hole in the wall. Small, loud and not for everyone.
Antigua Brewing Company
Craft brewery taproom a block off the central park, pouring its own beers plus guest taps, with a rooftop terrace that is one of the few places in town with a volcano view after dark.
Calle del Arco and the park blocks
The street under the Santa Catalina Arch and the blocks around Parque Central hold most of the bars, rooftops and late taco stands. Marimba and street food on the park most evenings, and everything within a few minutes' walk.
Salsa nights
Several bars run salsa and bachata evenings with free beginner classes early on, which pulls in Spanish-school students and locals in roughly equal numbers. The rotation of which bar hosts which night changes, so ask at your school or hostel rather than planning ahead.
Getting home on the cobbles
The streets empty quickly once bars close, lighting is dim and the cobblestones are genuinely ankle-breaking in the dark. Tuk-tuks circulate around the park until late and cost very little, so use one rather than walking any distance.