Since this morning, online ticketing for archaeological sites appears to be available for the 2025 summer season, at least for the Acropolis (but the website response times are terrible, maybe the system is still in the testing phase).
Official website:
https://hhticket.gr/tap_b2c_new/english/tap.exe?PM=P1N
Acropolis & Slopes tickets :€30.
Over 65 YO (EU): €15.
Up to 25 YO (EU): Free.
Up to 18 YO (Non-EU): Free.
The combo ticket has been discontinued.