The Acatenango overnight is the trip nearly everyone in Antigua is here for: five hours up through farmland and cloud forest to a camp at around 3,800 metres, facing Fuego across the col, and a night of watching it throw rock into the dark. OX runs that, along with the harder push onto Fuego's own flank, day trips up Pacaya, and mountain biking and kayaking when the volcanoes are closed in.
They publish an emergency number alongside the booking one, which tells you something about the terrain. The office is on 2a Calle Poniente; the coordinates here are the town centre rather than the doorstep.