Battambang Provincial Museum is small, but its collection is stronger than the building first suggests. The core exhibits are Khmer sculpture, lintels, and temple fragments from the Angkorian period, backed by prehistoric material from Laang Spean cave.
It is a useful stop before visiting Wat Banan or Phnom Sampeau, because the museum gives names, dates, and religious context to the stonework you will see in the countryside. Allow 45 minutes; the English labelling is generally enough to make the visit worthwhile.