Olympia is probably one of the most well known of all of the ancient sites in Greece. To get there from the Southern Peloponnese is a bit of a slog although there are organised tours from most major tourist resorts.
It’s a large site and rather muddled to the untrained eye [like mine]. There are piles of crumbled walls and pillars covering more than a thousand years of Greek and Roman history that make it hard to visualise the site in it’s heyday.
The Temple of Hera, the entrance to the famous Stadium, and the Temple of Zeus are impressive and the Archaeological Museum is excellent.