The 2nd worthwhile sidetrip from Nafplio was to go see Mycenae, and it was conveniently located on the drive out of Nafplio when I realized that I had already done everything in one day, and it would make sense to push to Olympia instead of going back to Athens directly.
Got there bright and early soon after it opened, and got to learn about the Myceanaean culture, which predated the greeks by about a thousand years:

That was an interesing 1.5H+ visit, and then time to drive to Olympia, a longish 3h+ drive...

The Athens Archeological Museum had a good amount of artifacts from this location, including a nice overview: