I will be arriving in Athens. Definitely want to see Delphi and the Meteora and the Corinth canal, possibly Nafplio area. How much time should I plan for this? I plan to leave the mainland to visit Santorini & possibly Crete.
It depends a bit on your pace and mode of travel. Delphi could be done on a daytrip from Athens, either on your own or one of the many tours; or you could overnight there. The Meteora has good train service from Athens to Kalambaka, I would plan on two nights, travelling up one day, a day to explore, then travelling back the next. You can do Delphi/Meteora as a route, as well, maybe saving a travel day. Nafplio and Corinth can be done on the same trip. Easy bus ride, get off right at the canal, catch the next bus to Nafplio. Nafplio deserves two nights minimum for an uninterupted day, add a night for Epidaurus, another night for Mycenae, using Nafplio as a base. Of course Athens deserves some time, but seeing you will be in and out of there, you can break up sites there as time allows. Main thing to remember is that any day you travel in Greece, is pretty much a travel day. You may have some time in the morning, or late afternoon, but Greece operates on it's own pace.
Here was my itenirary from June, hope it helps.
Arrived in Athens on Friday PM, did Athens and left on Monday AM in rented vehicle to Delphi, did Delphi and spent night in Delphi. Tuesday AM headed to Olympia and did the sights. Spent night in Olympia. Wed AM drove to Nafplio and spent 2 nights. Friday AM left Nafplio and drove to Athens (visited Corinth canal) for afternoon flight out. I don't know how long you have but this was our schedule and it worked well for us! good luck.