Sairecabur stands at 5,971 metres on the border with Bolivia, north-east of San Pedro, and it is the mountain guides use to find out whether you are ready for anything bigger. A mining track climbs to around 5,750 metres, which leaves a short scramble over loose volcanic blocks to the summit — perhaps an hour of walking, but an hour at an altitude where an hour is a long time.
The crater is broad and the rim is littered with the remains of a high-altitude astronomical observatory, abandoned after a few years of operation. From the top the view runs north along the whole Bolivian volcanic chain and south over Licancabur to the Salar de Atacama.
Do not treat the short walk as a reason to skip acclimatising. Going from San Pedro at 2,400 metres to nearly 6,000 in a morning is how people end up with a headache that follows them for three days. Most operators want you to have done Cerro Toco first.