Now you can get your Acropolis tickets ahead of time. You can get single tickets to anyone of the sites in central Athens or a combo ticket that includes all of them. Choose the day, choose the time (this is slightly flexible as you will see) and if you choose the combo you have 5 days to use all of the tickets.
Combo tickets are for the Acropolis and Slopes, Ancient Agora, Roman Agora, Hadrian's Library, Olympieion, Kerameikos, Aristotle's School [Lykeion]. Tap here for more info.