1. One day cruise, that is visiting 3 islands ( Hydra Poros Aegina)

2. Nafplio using a KTEL bus

3. Halkis with either a KTEL bus or a train

4. Marathon Lake,Marathon Tomb and Nea Makri ( on the sea). doable with a rental car, otherwise you can go to N. Makri by bus

5. go to the island of Aegina for the day

That's all i can think of, as for most other trips you would be needing a car......

Is it day trips you are interested on or 1-2 overnights elsewhere that Athens? If you consider one or two nights away,an alternative to Olympia might be Meteora to the other direction (maybe in combination to Delphi?)

As a side note, Athens offers many nice surrounding areas, depending on your interests of course. Besides the city center main sites, there are sites all over the place such as Vravron, Elefsis and the like or interesting museums such as the Piraeus Archeological one....On the south, Piraeus itself is very pleasant away from the port area with 3-4 different harbors, and the whole Attica coast from Piraeus till all the way to Sounion has something to offer. From a nice stroll to Flisvos to posh fish restaurants and shopping in Glyfada, to a dip in Vouliagmeni..... On the North,suburbs such as Kifisia,Ekali,Marousi offer nice strolls and pleasant architecture, and you may see how local rich and famous live and there is nice shopping, malls and pleasant squares and boulevards. Further north, even people without kids enjoy the Attica Zoo Park in Spata area near the airport. Near city center, each area has a different ambiance, or example if you take metro or bus to Papagou and you walk to the park there, you'll be found surrounded by pine trees and running water and you will not believe this is only 15 minutes away from Athens center. Even purely residential areas such as Moshato or Kallithea have a pleasant suburban feeling with quiet lanes and vibrant squares......
Do not underestimate Athens area.

We did a daytrip to Sounion by bus. It has a well-preserved temple of Poseidon on a fantastic sea view site.

And I could see a whole day in Hydra, by ferry from Piraeus. Hydra is a beautiful island, with no cars. Local transportation is on foot or by donkey. There are boats and windmills and beaches. (Pebbly shore or rocky platforms.)

Re: Delphi, considering taking a bus there from Athens in the late afternoon/evening. Stay at a Delphi hotel and get to site early, before the busloads. Then return to Athens in the afternoon.

Day trips are ideal for us if the journey is doable. Will not mind a overnight stay if required. Yea, think Nafplio seems like a gorgeous place to visit but 2d1n for Nafplio seems to do injustice to the place as many said that there are lots of places to visit around that area? Will explore the sub urban of Athens as well if we don't really feel like traveling far!

jopudding,
For what it's worth, my husband and I really enjoyed the day trip that clausar suggested...........the cruise that visits 3 islands.
We enjoyed the boat ride itself and loved walking around the islands that we stopped at, taking in all the views etc

There are lots of places to visit around Nafplion (which is beautiful), but many are close and can be done in the same day. Due to time limitations I hired a private driver who picked me up a 8am at my hotel in Athens. We visited, at my request, and at my pace, Mycenae, Tiryns, Nafplion, Ancient Epidavros and Ancient Corinth. I had a couple of hours in Nafplion to have lunch, a gelato and to visit the Palamidi and walk around the old area of town. I was back in my hotel by 6pm.

I loved Delphi too. If you go, think about spending the night in Arachova, about 20 minutes past Delphi. I thought it was more charming than Delphi Town.