11 day Greece itinerary recommendations?

Save all your time on the mainland for the end of your trip so you aren't accidentally stranded out on an island, due to an unforseen mechanical breakdown or extremely high winds, at the last minute for your flight home. When you first arrive at the Athens Airport take the first flight out you can get to the first island of your choice, then a flight or ferry to your next island or back to Athens.

Where in the Peloponnese did you have in mind? That's a pretty long drive for any day trips. Of course it depends on your flight schedule, but we drove upon arrival at ATH to Nafplio (two hours) and spent a few days there before making a circuit of the Peloponnese, ending with a night in Athens (we know we'll be back, and this was enough time to see the Acropolis and walk around the city a bit).
I don't know anything about boat charters, but you could do two mainland stops and have time to fly to Santorini and back.

Consider going to Nafplio for 2 full days. It is 1 ½-2 hours away from Athens, so making it a day trip is not appealing, and it offers enough history and culture to fill 2 full days and not want to drive back to Athens. Stop at the Corinth canal en route and walk the bridge. There is certainly a LOT more to do in the Peloponnese than Nafplio, but Nafplio is closest and easiest considering your 11 days (we spent 8 full days driving all around the Peloponnese and loved it and would go back for more. I think it gets short shrift). Then you could also do an overnight in Delphi, or make it a long day trip, and then still have 1 ½-2 days in Athens (we spent 1 ½ and would have liked ½ - 1 more).
We chartered a boat for a 5 hour trip around Santorini and that was enough for me (Im not a sit-on-a-boat and gawk type - my husband planned it - and I did enjoy it). It was with others, but Im sure you could find something similar for just yourselves if interested. They served dinner and we got a great perspective of the island, so it was worth it after all ;p We spent 2 ½ days/3 nights on Santorini and that was enough for me. It is a very polished, magazine version of Greece that people need to see to check some box, it seems, so while its nice to have done, it wasn't a highlight of the trip (many highlights in the Peloponnese and in Athens). We liked the variety and more authentic feel of Naxos, but you have only 11 days so adding it would stretch too much.
I agree with leaving Athens for last. There are flights to Santorini. We took ferries to islands for the novelty and fun. It was.

If you want to experience Santorini then fly there after you land in Athens. We spent 3 nights on the island at that was about right; however, we took the ferry but would not advise because it takes too long (about 8 hours). Then fly back to Athens for the remainder of your trip. Four to five days there will be good. So then if you still need more island time you can day trip or overnight in the Saronic Islands from Athens (Piraeus). In addition to Nafplio, as recommended by others, there is Mycenae (major archaeological site) nearby. Disclaimer - I've not visited the Saronic Islands or Nafplio but have had 2 trips reserved and cancelled to those areas. One due to a sudden strike and another due to COVID. Maybe someday...

Quick FYI...I understand it can be very windy in August causing cancellations for boat charters/cruises. Never been to Greece and looking hard at a trip there too. Have fun!

I agree with several previous contributors who urge you to save Athens until the END, and probably Nafplio also. Many reasons: (1) With only 11 days, every hour is precious, and going into ATHens 2x takes valuable time (2) you'll be too exhausted for a sightseeing marathon in a busy capital city; instead, recover & re-energize on the islands (3) island-first, you'll learn how Greece works, so later you can navigate Athens efficiently and (v. important) (4) after 8 days of immersion in history and culture & ancient sites, the famous Athens landmarks will resonate so much more.

Since you arrive late, here's a plan:
Night #1 --Hotel Avra, 25 mins away, has free 24/7 shuttle; go there direct from airport, have a late snack, early bedtime, up early & shuttle back to airport for Aegean flight 7/18 to Santorini -- 7am ticket w checked bag is $117, 10am is $128. (If u decide on this intereary, book NOW - you chose THE busiest costliest travel week of the summer !! - Greece's biggest holiday is Aug 15).
Night #2 - Arrivng Santorini early, you can walk along "famous caldera view" check the (Good) small museum in Fira, take a nap (u are still recovering) and have a sunset dinner -- book a rooftop table at Fira's STANI taverna for UNcrowded view & great prices. For a cliffside hotel walkable to Fira BUT bypassed by crowds, i recommend Firostephani - Hotel Mylos still has caldera-view balcony rooms for August 18; check this link - https://www.booking.com/city/gr/firostefani.html
Day #3 - See the "famous village" of OIA (bus or taxi at 8 am to beat package-tour & cruise mobs). At 3:15 pm, take large modern Blue Star ferry to a beautiful, less-crowded isle. (NOTE: buy ticket on day #2 because of busy week! ) Naxos or Paros or adorable Antiparos are all nearby. Any of these has day-excursions on small boats to explore other beaches or islands.

Niights 3-4-5-6 - Enjoy island time!
Day/Night #7 - FLY back to ATH (time is precious). Arrive by 11+, rent car, for lovely drive to Nafplio - en route u can stop an hour at NEMEA (wonderful Sacred Games Site!), that afternoon explore Old Town Nafplio, experience Greece's BEST sunset IMHO, from seafront cafes.
Day/night #8 - See MYCENAE? or (less-crowded) TIRYNS - cool off at Town beach or nearby Tolo
Day #9 - leave after breakfast, drive up back way of PALAMIDI, then head East to EPIDAURUS - mainly its fab theatre, then back to ATH airport return car, flat-rate taxi to central Athens hotel or apt (choose one CLOSE to Parthenon to be walkable to sites).
Night #9-10-11 - ATHENS sites

You may be surprised at my detailed advice but I've addressed this very same inquiry SO many times I developed a response that people have found very satisfying. I hope you'll give it a try!

Janet, this is incredibly helpful and sort of similar to the revised idea I’ve been kicking around in my mind. I like the idea of a short overnight in Athens on arrival and then flying to Santorini first thing in the morning. For my wife and I, I think we would to splurge for this part and stay three nights in Oia, and then move on for two nights in Naxos. Does this seem reasonable? Thereafter I definitely like to idea of staying two nights in Nafplio to enjoy that area and then finish up for the Athens sites.

You can splurge on the hotel part for Santorini ... and stay in OIA if you insist ... be aware that between 10 & 4:30 or so, and then from about 6:30 until 30 minutes after sunset, OIA will be crammed with tour groups. If you are relaxing on a balcony, you may find people peering at you... unless you've sequestered yourself at a spa with walls. The length of stay also depends on your interest in other parts of the island, or not, and whether you crave swims at a beach .... as u probably know from research, Santorini beaches are crushed black grit (lava?). For sandy beaches, the 3rd day should be Naxos... but if upscale jewelry & fashion shops are more of a priority, it's Santorini. (BTW, on similar trips I've always saved clothes & accessory shopping for Nafplio... it's the favorite getaway-spot for savvy athenians, and shops for women AND men's things are tres chic).

Have fun planning -- just remember: Rule #1 is BOOK essential domestic Flights BEFORE you book any rooms. You can always find another room, but you can't magically find another plane if the one you wanted is sold out. Promise me you'll do that, and all the rest will work out.