Greece for 7 or 10 days

I was also so intimidated by planning a trip to Greece because of the sheer choice. I had already spent a lot of time on the islands of Croatia, and I really wanted to see historical sites, so I stuck with the mainland and I could not have had a more successful trip. Here is how I would consider allotting the days:
If three, a trip to Nafplio, where in addition to the town itself, you can see Nemea, Corinth, Epidaurus, Mycenae, and more.
OR, you could see Delphi and Meteora with three days.
With six, I would still consider sticking to the mainland and see more of the Peloponnese (Messese, Mystras, the Mani), but with six days, you would have time for an island or two. Santorini would probably make the most sense because you can fly to expedite things, and then pair it with perhaps Naxos.
Please share what month the trip will occur, as that can affect recommendations. Also, you may have heard that right now Greece is suffering a terrible heat wave and concerning wildfires. Hopefully they will find relief soon.

I was headed in to make recommendations but would just second valadelphia - she is spot on. I will add that it may also make a difference whether or not you are willing to rent a car.

I would highly recommend spending your remaining time in Nafplio, one of Greece's nicest towns and a great place to base yourself to nearby world-class archeological sites.

If you have a car you can drive from Athens to Nafplio or take a bus, then rent a car in Nafplio.

Here's the best website for Nafplio filled with everything you need to know about Nafplio and the entire region:

http://www.visitnafplio.com/

Nothing wrong with islands but it's a lot easier getting to Nafplio and you have far more options on what to do, see and experience in the region.

travelmom makes a good point--without a car, you might have to scale back a bit. I'd take a look at the transit options before deciding, as I imagine if you are traveling solo you might prefer to not drive.

Yeah I’ll have to be at the mercy of public transportation unfortunately :/ haha. That might mean me having to be based out of Athens. How is transportation throughout the country?

I’ve always wanted to see Delphi and I just recently gotten on board with Meteora. I’ll be going in late June and I’ll be flying from Rome. My idea was to maybe fly to santorini first and spend 3 nights on the island, granting me two full days to explore and take it in. Then I can fly (or ferry? What option is easier? How’s transportation to the airport while on the island?) back to Athens and spend 3 or 4 nights? I originally gave myself 3 full days to explore the city. Is that sufficient or should I dial it back to two full days? I can potentially spend one of those 3 full days seeing Delphi as a day tour. That covers a packed week. Is there time to do something cool with 3 more days?

Three days in Athens is good. With 7 nights I'd add an island (we did Santorini - fabulous) and with 10 add Delphi/Meteora.

Meteora and Nafplio can both be reached by public transport, you just won't be able to stop places en route as easily. If you choose Meteora, I'd probably stop en route at Delphi for a night then stay two at Meteora (three might be overkill unless you are a keen hiker). Nafplio would also be great for three days, and I know bus there and back would be simple. You'll have to investigate how getting to Epidaurus or Mycenae would work from there, but I'm sure it is possible. If you saw neither of those, Nafplio would still be great for a few days. good luck!