Athens day trip

I wouldn't try Nafplio as a day trip from Athens, I'd want to spend at least one night there. In your case, that would mean paying twice, ouch! Consider maybe instead a day trip to Aegina, an island close to Athens and easily reached by ferry. When Janet reads your post, if she agrees it makes sense, she'll give you more particulars. (If she doesn't agree it makes sense, listen to her not me!)

Napflio is worth a minimum of two nights. Can’t you cancel your Athens apartment or reduce your nights in it?

You could consider contacting Dolphin Hellas or any other travel agent. They have one day bus tours to Nafplio which include Mycena and Epidavros. The bus picks yo up at your hotel. We took the two day tour which gave us a hotel in Nafplio.

I second stanbr's recommendation for the Nafplio bus trip -- with the proviso to do a 2-day ... if you try to cram everything into 1 day, you'll be on a bus most of the tme, and only a rushed lunch-hour in Nafplio... which is considered THE most beautiful Old Town in all of Greece. A crime to short-change it! Here is the BEST 2-day -- http://www.chatours.gr/tours.php?exc=8&exccat=1 because of how it's laid out. It covers the famous sites... AND, importantly, gives precious "free time" so you don't feel as if you're on a tour.... the whole afternoon & evening in Nafplio (the June sunsets from its seafront cafes are to die for!), and also next morning. It's listed as "Tuesday & Wednesday" ... not sure if this means 2 x a week Tue-Wed, and Wed-Thurs or just 1x. you can ask.

Two things to know:
(1) Ask your hotel about leaving/coming back -- Athens hotels are VERY used to people leaving/returning due to trips like these ... they'll stash your big bag in their luggage room & you can just take an overnighter. And -- unless you took a rock-bottom price that doesn't allow for ANY changes, with adequate notice -- you wouldn't be charged for the night away. If on the night you're away, they fill your room, they'll put you in an equivalent one on your return. That's been my experience.
(2) On the CHAT tour, there's a choice of "first-class" and "Tourist Class" hotel in Nafplio. Do NOT take 1st class -- it's outside of town & requires taxi back & forth. The "tourist class" is a perfectly fine hotel (Victoria) with balconies, just 1 block away from the Central Square... which is a perfect delight at night, kiddies riding trikes in the moonlight, on the MARBLE paved piazza.

Nafplio is beautiful for a day. You can start early morning and come back in the evening.
In Nafplio you can see the Palamidi which it is a fortress. There are two main approaches to the fortress. You can drive or take a taxi which probably cost about 10 euro or take the steps (576 steps to the outer gate and 901 steps to the entrance to the castle)-I recommend the steps.
I have been last year in Nafplio and I don't recommend the Bourtzi Castle, as is very dirty and looks like no one is taking care of it. ( a lot of pigeons)
The Bourtzi Castle is beautiful if you will see from the top but not to approach it.

From Athens to Nafplio is about 2 hours driving. You can rent a car from the airport or get someone to take you there.
On the way to Naflpio, you can stop to Corinth Canal for pictures (the canal separates the Greek mainland from the Peloponnese, turning it into an island) it' really awesome.

If you are flexible to change your hotel in Athens, you can also spend a night in Meteora which is one of the largest and most important complexes of Greek Orthodox monasteries in Greece, second only to Mount Athos.
Is really really beautiful if you like a nature beauty.

We liked Naflpio, we walked up the hill to the old fort.

However, not sure it is wise for a day trip from Athens.

Consider day trips from Athens to Delphi, Corinth and/or Sounion (Temple).