Olympia is a loooong drive for a day trip. Exactly what are the dates? And what have you read about either site? Research will help you decide what interests YOU most. If you happen to be in Greece at the end of May this year, you are VERY lucky - because you can see a Revival of an ancient Greek "Sacred Games" -- NEMEA -- less than 2 hours away from Athens.
The Ancient Nemea site is just over the Corinth Canal about 20 KM on the main road into the Pelopponese (See this map - https://tinyurl.com/zekuft9 -- cllick & it gets huuuge & you navigate using side-slides). A "runner-up Games site" to Ollympia during Classical Times, NEMEA was buried for more than 1500 years, but carefullly excavated, and authentically restored in the past 30+ years. Greeks themselves brought about the Revival of the Games, every 4 years, http://nemeangames.org/ May 29-30 2024, is the 8th Revival, attracting thousands of conttestants of all ages from around the world. The events and ceremonies follow traditional customs and is a thrilling spectacle .Even if your Greek visit is earlier, or afterward, there will be plenty of atmosphere and festivity surrounding this event.