As a couple, Delphi Day-Trip works.
Delphi's mountainside setting is breathtaking and the sense of standing at the 'navel of the world' where the oracle prophesied is profound — for ruins-and-myth lovers it's the best day-trip from Athens. The catch: it's a serious commitment (2.5–3 hours each way), the site is steep and exhausting in heat, and on a packed bus tour you're rushed through both ruins and museum.