Antigua is where Guatemala's adventure trips are sold, whether or not they happen here. The volcanoes are the reason: Acatenango stands over the town at 3,976 metres with Fuego erupting beside it, and the overnight camp on the ridge between them has become the thing people come to Guatemala to do. Pacaya, an easier half-day, is the other.
The market is crowded and the trips look identical on paper. What differs is group size, whether the guides are from the villages on the mountain, and what you are given to sleep in at 3,800 metres — it is genuinely cold up there, and hypothermia, not the lava, is what goes wrong. Ask what the tent and the sleeping bag are rated for before you pick on price.
The town is also the base for Lake Atitlán day trips and for cycling in the valleys, both of which are better organised from here than from the lake itself.