The Peloponnese is compact enough that several of its headline sites work well as day loops from a base in Nafplio, Kalamata, or Athens.
Modern Sparta, built on the site of the legendary military city-state, makes a low-key base for visiting nearby Mystras, the far more visually dramatic Byzantine ruins just a few kilometers away. The pairing lets visitors contrast classical Sparta's minimal surviving ruins with Mystras's extraordinarily preserved medieval townscape in the same day.
Greece's fifth-largest city gives its name to the famous Kalamata olive, and its old town, castle, and seaside promenade make an easy base for exploring nearby Messinian villages, olive groves, and the beaches around the Messinian Gulf. It is also the main transport hub for reaching Methoni, Koroni, and the Mani peninsula.
A longer but rewarding drive along the eastern Peloponnese coast to the fortified rock town of Monemvasia, combinable with a stop in the seaside town of Gefyra for lunch before or after exploring the medieval streets and clifftop fortress above.
A dedicated day trip to Ancient Olympia and its museum, typically reached via a scenic drive through the western Peloponnese's hills and vineyards, worthwhile even from a base as far away as Nafplio or Kalamata given the site's significance and size.