In some ways, Epidaurus can be viewed as two different sites in one - the sanctuary and the theatre.
The history of the sanctuary is fascinating and I found it really interesting to see how some of the principles of medicine that we take for granted these days were being used thousands of years ago - but within a religious framework. There is not much to see of the original buildings, but the general layout is here and the small museum has a decent collection of artefacts, so you can piece it all together.
Visually, the theatre is the highlight of visiting Epidaurus, and it really is a remarkable ancient monument. Sitting right at the top, you can hear someone talking down on the stage. I visited during the day and I regret not doing some research to see whether I could've attended a performance in the theatre in the evening (even if most of the festival is in Greek). I would recommend having a look into that before you visit.
Epidaurus is close enough to Athens that you can do it as a day trip, although it's a bit tricky by public transport because of the limited bus timetable. If you are coming by bus, you might be better off basing yourself in Nafplion for a night or two and seeing a few sites in the region. If you're using a car, or joining a tour from Athens, it also makes sense for travellers interested in WHS to also visit Mycenae and Tiryns, as well as the former TWHS at Corinth on the way through.