Not really an itinerary, just sharing a bit of info -
I spent about 10 days in Greece last year around April. Visited Athens, Mykonos, and Santorini. I preferred the islands to Athens, although the places suggested earlier are good. If you're flying between the islands, try to go for Aegean Airlines instead of Olympic Air. Aegean is cheaper and the service is better.
Bring sunglasses and a hat. The sun can be blinding at times, especially with all the whitewashed buildings.
In Santorini, try to make your way to Oia for the sunset at least once, it's breathtaking. Go early (about 6pm) to get a good photo spot. The place will be filled with fellow travellers and photographers. Dress warm though, as it gets very cold once the sun dips (around 8pm). If you drive, you might want to rent a car instead of relying on the infrequent public buses, or be prepared to walk a lot. Look out for the donkey dung, it's everywhere. Good, non-expensive, authentic food can be found at Nikolas' Restaurant. It's where the locals go.
In Mykonos, there are the rustic windmills and nice walks along the coast. If you are keen on a good hilltop view, try to stay in one of the windmillhouses. During our stay on Mykonos, we did a daytrip to the island of
Delos. According to Greek myth it's the sacred birthplace of Athena and Artemis. Really cool, mystical place filled with interesting statues. Try to take one of the guided walks. The guides are pretty good and you learn a lot about Greek history.
In Greece, some of the older hotels and guesthouses use the old type of electric sockets, so bring the appropriate adaptor for your chargers etc.
Most folks speak and can understand English, and some of the older folk are quite friendly and may strike up a casual conversation. My partner and I met an elderly man on a ferry on our way to Santorini, and he ended up showing us around during our stay there.
Some phrases are helpful to get by...
Yassou/ yassas - Hello, and goodbye
Parakaloh - Please, excuse me
Efharisto - Thank you
Have fun!
PS:
Link to pics