Buy in advance tickets

If you do not have one yet, get the Rick Steves guide to Greece or Athens. Another approach would be to visit the location on the ricksteves.com where Greece is discussed.

This kind of open-ended question does not have an answer. So much depends on your interests.

As to a May visit, I don't think crowds will be all that huge. Late May is different from early May. We visited in 2019, and had no trouble buying a combo ticket for all the big sites in morning of our visit - we went directly from the ticket office at the Parthenon up to the top of the mesa.

Thanks Paul...
I have the RS book but I have found out that even though RS recommends advance purchase some attractions will not be as crowded depending on the time of the year you travel. So in essence what I am really asking is "What has been your experience buy same day tickets for the popular attractions?"

I think we were there late May. Temps were fine. We walked up to the ticket office at the parthenon, saw 20 people in line, got our ticket - about 9:30. No need for advance tickets at that time. That was the big combo ticket for Agora, jupiter temple, etc. The only lines were at the National Archeological museum, which had 20-25 people in line.

For the summer period starting April 1st, tickets for the main archaeological sites will not be available until the end of March.

If you go in May, there’s no need to panic, the only archaeological site that requires advance reservation (by choosing a time slot and a date) is the Acropolis.

Tickets for all archaeological sites can be purchased on the official website:

https://hhticket.gr/tap_b2c_new/english/tap.exe?PM=P1N

If we are to believe the official announcements made recently, it is likely that the combo ticket will no longer exist from April 1st. It will be confirmed (or not) at that time.