Here is a pretty good description of the known rites of the agrarian mystery cult of Eleusis in Ancient Greece, which was centered around the farming Goddess Demeter and her daughter Persephone.

Although the penalty of revealing the rites was death, many early Christians did so anyway (it would seem as a way to combat the 'heresies'), so we have an account from Clement of Alexandria of the lesser rites performed, as well as quite a bit about the basic structure of the "epopteia" which was the grand rite.

The preliminary rites involved fasting, drinking of the "kykeon" (which is suspected to have been at least in some cases an intense psychedelic) and taking an object contained in a box and placing it in an open wicker basket (a common theme in agrarian cults). With this in mind consider that the chief priest was named the "Hierophantes" (one who shows important things, roughly) and it becomes clear that the mysteries themselves weren't the rites as performed, but rather were the sacred objects shown during the rites.

So what was the ultimate sacred object revealed during the Eleusinian cult celebration? Again we have a few accounts from Christian participants, one being Hyppolytus in his "Refutation of all Heresies" in which he gives a sarcastic recollection of the final rite: "the Athenians, while initiating people into the Eleusinian rites, likewise display to those who are being admitted to the highest grade at these mysteries, the mighty, and marvellous, and most perfect secret suitable for one initiated into the highest mystic truths: [I allude to] an ear of corn in silence reaped. But this ear of corn is also [considered] among the Athenians to constitute the perfect enormous illumination"

So there you go, it would seem that for the agrarian cult at Eleusis the ultimate sacred mystery was an ear of corn. Pretty fitting I suppose