When I think of giving thanks, I think of the Psalms.
Go to Erica Daniel's "Thanksgiving" blog entry at https://www.personalcreations....alms-of-thanksgiving for a list of "Psalms of Thanksgiving" and some wonderful graphics for each. She has a good list of other "thanksgiving scriptures" as well.
You could also explore some of the festivals from the Old Testament. There is a nice chart in my NIV study Bible (in Leviticus). Several feast days that seem appropriate to Thanksgiving are First Fruits (Lev. 23), Weeks/Harvest/Pentecost (Lev 23), and Tabernacles/Booths (Lev 23).