The intercity (KTEL) bus from Athens to Nafplio is pleasant, modern vehicles, reliable, and used by the full spectrum of society. Seats are reserved, upholstered & comfy, with footrests. A/C is good and the view is great. (Request seats on LH side outbound, for best view of Saronic Bay enroute). I've both driven & taken the bus to Nafplio & can recommend it Often people take the bus to/from Athens, but may rent a car in Nafplio for a day to visit some sights in the area; your hotelier can easily arrange it.
From the airport to the Kifissou Ave KTEL Terminal, there's an express bus X93 right outside the building, you buy ticket at a kiosk, Cost €6. It takes about 60-65 minutes. Alternatively you can taxi about 40-45 minutes, €50? The Nafplio express busses go hourly on the half-hour, takes 2.5 hours. When you enter station, Nafplio tickets are NOT sold in regular ticket room, but in a small room right beside the bus, which is stationed directly at the Terminal exit door; ask anybody.
This superb NONcommercial website http://www.visitnafplio.com has info on all the highlights of Nafplio, including beaches, walks, food, hotels... but the best part of all is just relaxing in the wonderful seafront cafes at sunset... it restoreth the soul.
I do love the town beach... you can walk at sealevel around the peninsula point to get there, great photo ops. Beach itself is stony, but a platform on one side has cafe & sunchairs & you can enter the water from there. Local busses hourly also go 15 minutes to nearby sandy TOLO beach, which will be crowded to the MAX in late july. However, here's a secret: have the Tolo Bus let you off at the ASINE crossroads, and walk about 200 yards down a small shady road to the ruin of ASINE, and a tiny cove beach (sometimes called Kastraki). You can see the Tolo crowds from there but this is pretty much a secret. There's ONE taverna overlooking it & you can change into swimwear in its WC. And afterwards, it's great for a sunset dinner on its canopied terrace. Don't tell anyone.