The Tambo Quirquincho occupies a restored 18th-century tambo — a colonial inn and market building — a few blocks from Plaza Murillo. The museum's collection focuses on the art of Bolivian carnival: elaborate masks used in the Oruro carnival and the Gran Poder procession, historic photographs of festivals, and an interesting section on carnival costume evolution over the past century. The building itself, with its courtyard and original stonework, is worth seeing.
Opening hours are limited (Tuesday to Friday afternoons, Saturday mornings) and admission is inexpensive. It's a manageable 45 minutes and pairs well with a walk through Calle Jaén, which is nearby. The mask collection in particular is remarkable — the workmanship in the best pieces is comparable to anything in the folk art museums of Europe, with none of the crowds.