Overnight trip from Athens
 

We have scheduled April 30 - May 1 for an overnight trip to the Peloponnese from Athens. Just learned from calling Key tours that they won't be operating because May 1 is a Greek Holiday. What are some other options available to us? We would rather not rent a car.

Planning on hitting some of the obvious sights, Nafplion, olympia, Epidaurus, etc...

Public bus from Athens to Nafplion, then local buses and/or taxis to near by sites on arrival day. May 1rst is a Bank Holiday and museums/archeological sites are closed. Nafplion town itsself is very pleasant though, and deserves at least one day, so you might want to take last bus back to Athens.
Frequent bus services, comfortable air conditioned coaches, about 2 hours bus ride.
http://www.ktel-argolidas.gr/?module...s_id=9&lang=en

Best way to see as much as possible would be to rent a car...
Taking the bus to Nafplio, would allow you to only see the city of Nafplio, which is very beautiful.
Renting the car would make it possible to visit Ancient Corinth, Epidaurus Nafplio and Mycenae with Tiryns, they all are in the same area and on your way.
Olympia is located on the west side of the Peloponnese , 204 km and about 3,5 hours away from Nafplio.

Forgot to mention that skipping Olympia, you could visit the archeological sites on April the 30th and save Nafplio for the next :)

Clausar and Marina, thanks for your helpful suggestions. May I ask you opinion on the Amphitryon hotel in Nafplion? Are there other nice luxury hotels I should consider?

Amphitryon is an excellent choice, it's one of the top hotels of Nafplio.
You could also have a look at it's sister hotel Nafplia Palace, but be warned... it is a totally overpriced hotel.....

Both hotels have an excellent view.....