Need to know what would be MUST SEE doable day trips using Nafplio as home base for 3 days--will be visiting Delphi on the way from Athens to Nafplio and Corinth on the way back to Athens.

Mycenae is fabulous and also Epidauros where there's a perfect amphitheatre.
http://www.travelmagpie.com

While you do Corinth be sure to visit Ancient Corinth, Perhaps Acrocorinth and Ancient Nemia a sister city to Olympia with a stadium, an excellent museum and a partially restored temple.
From Nauplion you can make a long day trip to Mystras the last home of the Byzantines. The city is spread down a mountinside and takes at least 4 hours to see.
Don't miss Ancient Tyirns just 4km outside of Nauplion it was the home of Heracules. Also remember to reserve some time to visit Nauplion itself. You could easily immurse yourself in history right there for 2 days.
Here is that trip in photos
http://travel.webshots.com/album/570747214TsFWOw

You could consider a day trip to Monemvasia

http://en.wikipedia.org/wiki/Monemvasia

Chania--on the map I have it appears that Monemvasia is quite a ways away-- how long of a drive is it from Nafplion???

IT is NOT a good day trip... overnight required.

you MUST VISIT epidavros, the ancient theater with the great sound, mykinai, the ancient city of the king, and ancient Olympia, the place and the stadium of the olympic games !!!

I have included Mycenae, Tiryns. Epidaurus, Nemea, Corinth as possible day trips since they are pretty close. Possibly Sparta and Mystras but they also look pretty far away. I have ruled out Monemvasia as it is too long a trip from Nafplion for one day--Olympia looks to be long too...how long a drive is it to there from my home base of Nafplion??? Is it a winding mountainous road there????

Yes sorry, it is a bit far for a day trip to go to Monemvasia. google maps shows it as 3 hours one way. In my mind, it was closer but I was coming from Gythio.

Are you planning to do some winetasting in the Nemea area? They have some excellent wines. We have very fond memories of this winery: http://www.lafkiotis.gr/ Great wine, relaxed atmosphere -- very friendly tasting.

With just three days in Nafplion, I recommend you concentrate on the closer sites like Mycenae and Epidaurus (and the wine!).