For 38 years I was one of those New Yorkers with no driver's license .. and sometimes travelled with drivers who didn't like driving. So I know all about how to get to Nafplio and around, by bus OR car. I think your best bet is a combo as follows:
BUS -- Of course by now you must have discovered the wonderful noncommercial website for Nafplio & the entire Argolid area - http://www.visitnafplio.com/ On its opening page at the top is a topics list including TRAVEL - DIRECTIONS - http://www.visitnafplio.com/directions.html This tells clearly how to reach Nafplio directly from Airport (either by rental car or by Xbus to the KETL Kifissou St. Intercity Bus Station)... and also from Central Athens Hotel to KTEL station by local bus ... however that's not convenient; far easier for 4 of you to taxi (have your hotel get the cab & tell the driver in front of you what the approximate fare will be... in case he'd get ideas about a roundabout route; should be? €20-22?)
The VISIT website's KTEL timetable is pre-covid; busses now are every 2-3 hrs, but you probably don't need to buy tickets ahead except maybe for Friday afternoon outbound and Sunday inbound. Have your Hotel desk call ahead for exact departures & plan to be there 30 mins ahead of time. Fare is about €15 one-way, may be slightly less round-trip. At Nafplio station u should buy return ticket ahead for specific return bus -- you can easily exchange it if you change your minds.
BUS NOTE: If you're taking 1st morning bus, bus station cafe is OK for take-out coffee, but otherwise typically grungy, so bring any snax. NOTE #2: Unlike other lines which use a ticketing room, Nafplio Bus has its own ticket desk right beside bus which is at a departure door (ask an employee "Nafplio" and he'll point). Busses are modern, upholstered comfy reserved seats (ask for LH seats outbound for best sea views). NOTE #3 Ride takes about 2 1/2 hrs, no bathroom on bus, but rest stop in 1 hour just across corinth canal. Final NOTE: When you get off, In Nafplio Old TOwn, you can taxi to an Old Town hotel for about €7? 8? depending. And my final tip -- if you think you may want to recover from journey on hotel balcony & have wine & snacks looking at view ... then Before you get taxi, walk about 200 yards toward seafront for a GREAT little supermarket full of cheese, wine, pastries treats, & then taxi.
CAR: FIrst of course spend at least a half a day on arrival just enjoying Old Town, enjoying the BEST sunset in Greece from a seafront promenade cafe, and planning your sightseeing schedule; After that, think about local car rental for ?2 days? (it helps to be specific about how many days u have!). Unless you have a week, local busses are too infrequent & not coordinated, so you couldnt do more than 1 site per day. With car, you could do 2 or even 3. A Nafplio "vet" recommends Staikos Car rental for good cars, good prices. Go on arrival, or ask hotel to book ahead, especially if you want automatic shift, less common. Here's the deal -- driving is easy, safe, low-stress around Nafplio... local 2-lane roads low traffic in October. And tell your reluctant-driver friend that SHE will pay nothing, you other 3 will pay. And you will buy her treats constantly, praise her, make her bed etc.
CAR NOTE: BROWSE This fab map - https://tinyurl.com/zekuft9 -- click & it gets huuuge; navigate with side-slides. You can see Easy places to drive to & pair ---(1) Ancient NEMEA (a llittle OLympia - thirlls!)... about 35 mins & then either MYCENAE or TIRYNS. (2) early start, drive up BACK Of Palamidi, then find road EAST across Peloponnese thumb to EPIDAURUS, on drive back if it's a warm day stop at a Beach - TOLO or KASTRAKI. As for MYSTRAS, (just west of SParta), you could manage in a day trip If you start early. Monemvasia would mean an overnight stay. And remember -- many sites/museums close at 3 in Autumn.
