Even on a tight schedule, Nafplio (& Mycenae/Epidaurus) Day-Trip earns the hours.
Allow Full day (10–12 hours).
Nafplio is the prettiest town near Athens — Venetian fortresses, neoclassical streets, a romantic harbour — and pairing it with Mycenae's Lion Gate and Epidaurus' flawless acoustics makes a rich Peloponnese day. The catch: it's ~2 hours each way and the classic tour crams three big sites into one day, so you skim each; Nafplio itself deserves an overnight.