The museum sits directly inside the sanctuary precinct and is included in the site admission. It is small, well-organized, and full of finds that make the abstract history of the Mysteries concrete.
The standout piece is the Niinnion Tablet, a 4th-century BCE painted terracotta plaque that offers the closest thing we have to a visual record of the Mysteries themselves — though scholars still argue about exactly what the figures depicted are doing. The collection also includes the famous proto-Attic amphora showing Odysseus blinding the Cyclops Polyphemus, one of the earliest narrative scenes in Greek art. There are votive reliefs showing Demeter, Persephone, and Triptolemos (the first man to whom Demeter gave the gift of grain), as well as architectural fragments, cult vessels, and a scale model of the sanctuary at its Roman-period height.
The model is particularly useful — it helps you read the ruins outside and understand which foundations belong to which phase of construction. Spend time here before walking the site.