Any suggestions on things to do that won't be too exhausting. They are still active, but I am afraid will tire easily.
In Athens there are a couple of companies who run little tourist trains in a circuit round the Acropolis. This takes you through Plaka, the pedestrianised tourist area, and then round the back of the Acropolis for great views. One of these starts at the top of Ermou, just on Syntagma Square. There is a metro stop at Syntagma.
There's also a hop-on hop-off bus which might be worth considering.
If they prefer walking then I would suggest a walk starting from Acropolis metro and ending at Thisseon. This is entirely pedestrianised and takes you under the Acropolis, the Theatre of Dionysus and the Herodion Theatre.
The walk would take me about 15 minutes. Initially it's a reasonably gradual uphill slope but from about a third of the way round it is all downhill. There are some good cafes for a break, coffee, and a superb view about two thirds of the way round.
It's not a suggestion, but a head's-up. There are spots in Athens where the walking surface is marble, which can be VERY slippery when wet. Don't want your parents slipping on it.
It has been some time but my recollection is that the historic Olympia site is mostly flat and not terribly large - probably .75 the size of a football field. A guide really helps here. We were able to tag onto a bus tour group. We offered to pay the leader but he declined.
Jennifer I hate to tell you but I'm your parents age, but I still run around like crazy. The Great Secret of Greece is the Late Afternoon Nap!! About 4 pm, everything gets quiet. Shops close. Get back to your hotel & lie down for 90 minutes... and PRESTO! full of vim for dinner & strolling in the Plaka district.
About that choo-choo on wheels; great, but do it after 6 pm otherwise it gets caught in traffic. Re Athens Museum -- instead of the Nat. Arch. Museum, do the BENAKI. Superb selections but very concise... won't exhaust. AND it has a divine place to have lunch or coffee. RE: Acropolis -- go to the NEw Acropolis Museum either just before or just after going Up on th Rock. This year we're going to the museum at about 3:30... then up to the rock about 6 pm... it's open until 7:30 and the crowds are gone.
RE Olympia: How are you getting there?? The KTEL bus takes 5 hours but is comfy reserved seats. Time it to get there for dinner & O'night. I stayed at Hotel Hercules. BIG TIP: the tour-bus mobs always go to the outdoor area first, then the Museum. YOU do the opposite: Hit the museum at 8:15 and you'll be all alone! PS: Museum impressed me more than the outdoor site. Are u going there & straight back to Athens? Seems a waste. Are you considering a tour?