Ancient Poseidonia has some of the best-preserved Greek temples in Europe. The three temples: Ceres, Hera, and Neptune are magnificent, and the excavation site is positively beautiful. We visited during the off-season, so we had the site and its museum almost all to ourselves.
The Paestum Museum is fantastic, too. (Admission is included in the excavation site entrance fee). It displays the pottery, jewelry, friezes, frescoes, sculpture and military armor - along with some Neolithic artifacts - found on site.
We drove to Paestum, which is far easier and less expensive than local transport. It cost 2.5 Euros each way, and parking at the site was free.
Travel time: 2.5 hours
Time on site: 2.5 hours
Total cost: 40 Euros (transportation, entrance fees, and food)