On a budget, Nafplio (& Mycenae/Epidaurus) Day-Trip still earns its price.
Tours €80–120; or self-drive/bus
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.