On a budget, Temple of Olympian Zeus & Hadrian's Arch still earns its price.
€8 (summer) / €4 winter; included in €30 combo ticket
The surviving 15-or-so columns are genuinely enormous and humbling up close, with the Acropolis as a backdrop. The catch: only a fraction of the original 104 columns survive, the field is small, and you can read the whole thing from the fence and Hadrian's Arch for free in five minutes — so the ticket is a tougher sell.