How long will you be in Greece?
Santorini is the best of the Greek Isles, period. Go there. Stay in Fira, the main town, rent a scooter and drive all over the island. Naxos is the largest of the Cyclaedes and is a great island if you want some peace and quiet. Mykonos is fun, but gets on my nerves pretty fast.
Athens is great. Find a pension or a hostel in the Plaka area, near the Acropolis.
If you want to go somewhere really cool, but that's tough to get to, go to Monemvasia. Google it. Amazing.
I thought Delphi was overrated, but I guess you have to see it. Further north, Meteora is incredible. A bunch of monestaries built on rock spires. Incredible landscape and hiking.
Sparta sounds cool, but it's not. There's nothing there anymore but a boring, modern city. However, nearby Mystras - a Byzantine ruins on the hill - is really cool.
Tell me where you're going and I can give you some more specific advice. Greece is awesome. It's my favorite country outside the US of A.
And yes, a gyro and french fry combo, maybe a greek salad on the side, is pretty much the best meal on the planet.