Many Hellenists live by the circular practice of kharis: religious reciprocity between God and man. Reciprocity might hint at a symmetrical relationship, but even the ancient Hellenes knew our connection to the Theoi is fundamentally asymmetrical--we can never hope to repay the many gifts bestowed upon us by the Gods. That said, if we assume our relationship to the Theoi to be asymmetrical, then the word and practice of kharis takes on new meaning: not just to reciprocate, but to give thanks.

In general, I think it is fair to say that the ancient Hellenes performed rituals and sacrifices for three reasons: they could be offered to propitiate the Gods, to ask for something, or for thanksgiving. I would even say that these three followed one after the other in a circular patters where the Gods were propitiated, petitioned for aid, and then thanked afterwards, when the gift was granted. Sometimes there would also be rituals simply to thank for the many gifts bestowed upon a person--especially those gifts that were never explicitly requested. Xenophon has a beautiful example of this in Cyropaedia:

"Zeus, god of my fathers, and thou, O Sun, and all ye gods, accept this sacrifice, my offering for many a noble enterprise, and suffer me to thank you for the grace ye have shown me, telling me all my life, by victims and by signs from heaven, by birds and by the voices of men, what things I ought to do and what I ought to refrain from doing. Deep is my thankfulness that I was able to recognize your care, and never lifted up my heart too high even in my prosperity. I beseech you now to bless my children also, and my wife, and my friends, and my fatherland; and for myself, may my death be as my life has been." [VIII.C7]

There are many ways to show gratitude to the Theoi. The giving of gifts is one way. Many votives and thank-offerings were left at temples and sanctuaries. The sanctuaries and temples themselves could be considered gifts. Votives were gifts promised to a Theos or Theia in advance--so in relation to a vow made to a Theos or Theia--while thank-offerings were given in relation to an unexpected event, or as a sign of long-term devotion. Sacrifices--be they of animals, fruit, or cakes--were also considered gifts and just before the quote from Xenophon above it is mentioned that:

"Straightway he took victims and offered sacrifice to Zeus, the god of his fathers, and to the Sun, and all the other gods, on the high places where the Persians sacrifice, and then he made this prayer."

There is no reason to assume that gifts are only physical. The actual prayer, as well as hymns that were performed, and music that was played, and dances that were danced, were just as much gifts as votives, wine, and meat. Anything given or spoken aloud within the boundaries of standard ritual practice would have most likely been considered a gift.

A while ago, I had a chance to perform a ritual of thanksgiving for blessings I had not noticed at the time, but which added up to a great blessing, indeed. Yesterday, I was lucky enough to feel the need to give one again, alongside a great friend of mine. I want to share part of a prayer with you today, in the hope that it will inspire you to look over your life and see if there is anything there that you have not yet given thanks to the Theoi for. I feel there is no kharis without gratitude, and that there are few things more fulfilling than thanking those who have blessed you so.

"Blessed Zeus, King of the Gods, accept our prayers and the kharis with them, and if it was not to You we should address our prayers, carry these words forth unto snowy Olympos, and let not our prayers and sacrifice fall upon deaf ears. 

Almighty Zeus: Councilor, Fulfiller, Savior; may you accept the kharis and words of your supplicants and our offerings in gratitude and as a token of our love for Thee and all the Theoi: 
To You we direct these words of humble gratitude: You who saw fit to bestow so many blessings upon your mortal supplicants. We offer you sweet wine and incense of blessed mien, for time has opened our eyes to all you have granted us as we take up and continue Your worship, but also before when we questioned and searched. You have led us to the worship of the Blessed Theoi and guided us in honoring Them, whom we were meant to serve from birth. You molded our fate so we would meet as kindred spirits of one mind and one heart who prays to You, son of Kronos, King of the Gods. You have granted us love, acceptance, and knowledge, in so many ways. You who have placed us, always, although we do not always see, onto the path of greatness and accomplishment, accept this humble sacrifice. These offerings would never measure against all You and Yours have given us, yet, these offerings are given with love, and in pious dedication. With our eternal gratitude, and in the deep knowledge of the Blessed Divine."