I may be biased. We’ve probably been to Nafplio twenty times over the years, often for a couple of weeks. It’s a great place to wander and soak up ambience. As well as a beautiful town and a massive fort it has a brilliant walk round the headland and, as you say, easy access to a lot of sites. It also has a good range of restaurants. The crucial thing for me though is that it’s a working Greek town with its own life and rhythm: it isn’t just a tourist destination.
We go back, get an apartment, and slot into life there: walking, swimming, trips out and just generally wandering about. In the evenings we go to old favourite restaurants or try out new places and there’s plenty of choice.
I went a couple of times with my godson and one of his friends when they were in their early teens and they liked the fact the town square and main restaurant street were traffic free and so we could let them wander a bit once they’d wolfed down their food.
If that sounds like what you’re looking for then Nafplio is an excellent choice.
PS I realised I started by saying I was biased but I didn’t know I’d love it so much when we first went. So it’s not bias it’s experience :-)