We are scheduled for the Greece 14 day tour May 31-June 13. I understand the tour includes the Acropolis, the Acropolis Museum and the National Archaeological Museum. We are arriving several days early and we want to tour other sites like Ancient Agora, Hadrian’sLibrary, etc.etc. It appears most sites require timed entry, each ticket is separate, and that you should pre buy tickets individually on the government website. Is this correct or is there any kind of combination ticket available?
Yes, that's correct.
Except for the Acropolis & Slopes ticket, which must be purchased by specifying a date and time slot, there's no need to buy tickets in advance for the other sites.
At the Agoras, Hadrian's Library, the NAM, etc, you won't have to wait, or at worst only a few minutes.
The Acropolis Museum is privately managed and has a dedicated online ticketing system.
https://www.theacropolismuseum.gr/en
Buying "Pass"-style tickets sold by private resellers like Get Your Guide or Viator is pointless and more expensive.
A reminder about "Skip the Line" offers: