Traveling to the Greek Islands
When travelling on to the Greek islands (after 72 hours) a self-test must be done (not older than 24 hours). Each transfer to another island (after 24 hours) must be retested.
To do this, a self-test form must be completed.
At the border there are samples with rapid tests: with a positive result you have to quarantine for 10 days at the expense of the Greek state
for more info: Travel.gov.gr.