Regarding getting to the Kiffissou Street KTEL (intercity) bus station for intercity busses to Nafplio... the X93 express bus leaves the air terminal bout every 30 minutes, takes about 60-70 minutes, costs €6... a taxi takes about 45 mins but could cost you €45. From a central athens hotel, a taxi to that station is about €15, takes 20 minutes. The KTEL express bus to Nafplio costs about €15, in normal times leaves hourly on the half-hour and takes about 2.5 hours -- however, during pandemic I heard it was every 2 hours; you can have a Greek-speaking family member in Zakynthos telephone this station to confirm the current schedule. THe busses are modern, reserved seats, comfortable upholstered w headrest/footrests, a/c, they are prompt, keep to schedule, because Greece has little rail service, KTEL busses are a very respectable form of intercity transit, used by people from all walks of life. The ride is pleasant, if u sit on LH side, u have a sea view much of the way.
I'm with Stanbr on staying in Nafplio... its Old Town area is at the tip of a peninsula, surrounded by the sea, so you feel as fi u are in a beautiful town on an island... AND wonderful swims available In town (2 spots) or other a 15 minute local bus-ride away. I urge you to look at this superb NONcommercial website - http://www.visitnafplio.com/ - and click EVEY link, including "Local walks" and Beaches. Its editor is a notable photographer herself, takes breathtaking photos of every aspect of the area -- you can check out her latest on Facebooks "Visit Nafplio" to get inspired. Yes u can do a daytrip to Hydra, but there u are limited by how far u can walk in a day. In Nafplio u can do the whol e surroundings without a car, and then rent a car for a day only, if u wish, to do a "ruin-a-thon" (Mycenae - Tiryns - Epidaurus in a day) Or do as we do -- ruins in the AM, beach in afternoon!