With only 5-6 nights (!) I feel your itinerary is drastically over-stuffed; Of course you want to see Everything--- but a cruel drive-a-thon = exhaustion, temper fits, everything's a blur. I beg of you, edit!! Have a Guidebook??? for $10 used on Amazon, ROugh Guide will insert a reality check, i.e., Sparta, it says no ancient vestiges remain of ancient days. Also, pleeeeze, x out Monemvasia! You'll spend all 5 days in the car!
PLANNING TOOLS - Again, a guidebook! and This FAB map (click to enlarge - use sidebars to navigate) http://euro-map.com/karty-grecii/peloponnes/podrobnaya-turisticheskaya-karta-peloponnesa.jpg -- is wonderful for visualizing an itinerary... all the way from Athens (or ATH airport) and back! BTW, save Athens for last day, since u need to be there last night before flight. And BTW, for this drive, use paper map in addition to GPS; the latter can lead you astray -- due to duplicated names + spellling differences. Based on my trips in Peloponnese, car & bus some comments & ideas.
OLYMPIA -- Didn't see until trip #7, because of long trek; 6+ hours, 2 busses. Museum is well-signed but ruins v. confusing unless you are a true ancient hist buff/scholar like moi. And the drive takes precious hours/days you don't have. A great Alternative is just past Corinth Canal -- NEMEA, "runner-up" to Olympia for ancient games, authentically excavated/preserved by U of Cal Berkeley. Mighty columns, temple ruins, museum -- and a Stunning stadium, with "heroes tunnel" and starting-blocks from 400 BC your men can race from! Everything but tour mobs!, and just off the highway! Show this to your guys - https://nemeangames.org -- 10 mins off highway, do-able in 1 hr.
NAFPLIO - on your list, looks like a lunch stop, but actually an IDEAL base for 3 days exploring Argolid ... the Most-Densely Historic area in Greece! This NONcommercial website http://www.visitnafplio.com - is info-crammed (click every link!!) describes sights, beaches, gorgeous Old Town & around. Nearby ruins: Iliad-era FOrtresses Mycenae, Tiryns, Asine (my secret gem), the town's Acronafplia, Towering Palamidi, etc etc. This photo album http://www.flickr.com/photos/stanbr54/sets/72157632094108982/ - shows highlites, & sunsets that outdo Santorini, IMHO. With this start, here's my suggestion for do-able rewarding itinerary:
DAY 1 (Arrive fr Santorini @ AIRPORT?) - Rental car on modern highway (see map), zip past Canal, exit @ Ancient Nemea sign, 5 km away. It's heart of wine country;optional stop @ Nemea Village bottling plant for samples (adults only!), Cross highway, go S 30 minutes to NAFPLIO by mid day. Explore Old town, go up AcroNafplia, OR to Town beach for late-day swim; sunset at seafront!
DAY 2 - Start w. Palamidi (drive up back, 1 parent walks DOWN 999 steps with teens, other parent drives to cafe at foot of steps to await them). Amazing Small Museum in Main Square -- a must for your hero buffs! 30-min video explains ancient wars, PLUS the place has world's ONLY Iliad-era suit of armor. More ruins? Mycenae is tour-mobbed, but 5 Km from Town, Tiryns is another HUGE fortress, almost deserted.
Day 3 - More exploring; TOLO beach, OR escape July Crowds just down the road -- ASINE fortress on Kastraki beach (only 1 taverna, great for snax or sunset dinner!)
Day 4 - Leave early, drive to Epidaurus (mainly for theatre) ... go to beach road (quick swim?) drive north, exit at ISTHMUS, to walk on Old Bridge for Canal photos (so deep! so skinny!). You can be at your hotel by late-day, go to Acropolis at 5 pm (crowds will be gone... wonderful golden light for pix!).
Day 5 - Other Landmarks - Ancient Agora. Acropolis Museum (3rd floor has GREAT video, photo & animated, of Parthenon history, downfall, restoring). Also 1896 Olympic Stadium (built on actual site of ancient one).