Church of Agios Spiridonas
A wonderful church and simultaneously the most important one for the city of Nafplion. Decorated with glorious byzantine paintings with beautiful colors and unique representations. The church is mostly recognized for an incident that took place in 1831, when the first governor of Greece Ioannis Kapodistrias was assassinated by two men while he had visited the church. The first bullet that did not manage to kill him, was stuck into a wall and remains there preserved until today (in a glass frame). This was the first political assassination that ever took place in Greece.
Nafplion is a wonderful town and a True Greece example. I would suggest you to enter the port in the afternoon, walk in the alleys of the old town, have dinner there and visit the Palamidi fortress early in the morning since after noontime the heat is really unbearable. Visit the castles and then hop onto your yacht and head to one of the beaches since there is no breeze in the port to cool down the atmosphere
-Eugene suggests