The Archaeological Museum on Koryzis Square holds the finds from the Sanctuary of Poseidon — inscribed decree stones from the inter-state amphictyony, terracotta votives, pottery, and architectural fragments. The collection is modest but well-presented, and the inscriptions (with translations) give a sense of how the sanctuary operated as a political as well as religious institution across several centuries.
The museum building is neoclassical and mercifully cool in summer. Entry is cheap. It takes about 45 minutes to see everything and is a good complement to a visit to the sanctuary itself — it's easier to read the landscape of the hilltop site once you have seen what was found there.