My trip to Greece is to experience history. We're going to spend around 6 days (this date is flexible) on the mainland. Then we're going to take a cruise. We'll go to Ephesus, Olympia, Mycenae, Corinth, and possibly Delos. On our own we were going to go to Delphi and Thebes. We'll get a combo pass in Athens and go to the archaeological sites there.
These are some of the most popular sites. Are they actually the hype? Again, this trip is for history, so there's some leeway on things like crowdedness.
Is there any more I should add? Especially places like the Peloponnese or northern Greece. It's 2-4 people and we're all able-bodied.
The major archeological sites are definitely worth going to.
However, many others will be mostly foundations, walls, a few columns here or there and you have to be a real history buff to appreciate but for me they are still worth going to.
Your best bang-for-your-buck is the Peloponnese that has more archeological/historic sites than anywhere in Greece. Other than Crete which has more than any other island the Peloponnese is your best option for what you are looking for.
I would recommend getting the Cadogan Guide to the Peloponnese as it's the best guide for that area. Filled with more info than other guide. It's available as a used book on websites like abebooks or ebay.
However with just 6 days you'll really be limited on how much you can see.
You could base yourself in Nafplio in the Peloponnese which is one of Greece's prettiest towns and is perfect for exploring nearby world class archeological/historic sites.
Here's the best website for Nafplio and you'll find you'll have more than enough to experience:
How much time are you planning for Athens? If you really only have 6 days for Athens and the mainland you are going to be pretty limited in how much you can see.
Athens needs 2 days at a minimum. That leaves you with 4 days for mainland exploration well 3 days actually because you need to be back in Athens the day before your cruise.
The suggestion from tommy is likely your best option as there is lots to see in and around Nafplio. You might want to go to Delphi (a great site) which is just a couple of hours from Nafplio.
If you can find a few more days for your land portion you will have scope for additional sites such as Monemvassia and Mystras.
Some images
Nafplio https://www.flickr.com/photos/stanbr54/albums/72157632094108982
Delphi and Meteora https://www.flickr.com/photos/stanbr54/albums/72157698930270992
Peloponnese, Mani and Monemvassia. https://www.flickr.com/photos/stanbr54/albums/72157711323535897
Have you already booked your cruise? the Mycenae, Epidavros and Corinth are very close to lovely Napflion.
Honestly, you will have more access to antiquities on a land based trip not a cruise. We saw everything on your list with a rental car and could time our arrival at those places to avoid crowds. We could linger at them if we wished
Take ferries to Mykonos and then on to Delos. All the others are very easy to access by rental car.
All of the sites you list are more than worth the hype if you aren’t in a cruise ship mob.