Where Does Thanksgiving Grow? is a neat data set produced by Linda Zellmer at Western Illinois State University. The data sets contain information about where the main ingredients in Thanksgiving meal come from. The data sets are displayed on maps showing you which states produce the most and least of each ingredient. For example, click on the turkey production data set and you will see that in 2007 Minnesota and North Carolina were the leading producers of turkeys. You can access the data sets individually or as a comprehensive PDF poster.
If you're looking for a writing activity to do with the students in advance of Thanksgiving, National Geographic Kids offers a Mad Libs-like story writing activity. Funny Fill-In generates a funny Thanksgiving story based on the words that kids write in response to Thanksgiving prompts. Quiz Your Noodle is another fun Thanksgiving game available on National Geographic Kids.
James Hollis at Teachers Love SMART Boards has developed an excellent list of Thanksgiving lessons that can be done using a SMART Board or other interactive whiteboard.
ABC Teach has numerous free lesson plans, coloring pages, and offline games that are designed for elementary school use.
History.com has a dozen videos related to the origins and history of Thanksgiving as well as video about current Thanksgiving traditions. Below I've embedded History of Thanksgiving, but I also recommend watching Mayflower Deconstructed.
The Year We Had Two Thanksgivings tells the story of Thanksgiving 1939. In 1939 Thanksgiving was going to fall on the last day of November which caused merchants to be worried about a shortened shopping season. In response to this concern President Roosevelt proclaimed that Thanksgiving would be moved up one week. Some states chose to ignore this proclamation and celebrate Thanksgiving on the last day of the month anyway. The conflict was finally resolved in 1941 when Congress passed a law stating that Thanksgiving would always be celebrated on the fourth Thursday of the month. The Year We Had Two Thanksgivings is supported by ten primary source documents. Included in those documents are letters from merchants appealing to FDR to change the day of Thanksgiving and letters opposing the change.
PBS Teachers has two lesson plans/ sets of activities about Thanksgiving that correspond to a classroom viewing of the American Experience series We Shall Remain. You can watch individual episodes or the entire series here.
Last, but certainly not least, Larry Ferlazzo has an extensive list of Thanksgiving lesson resources. In general, if there is a holiday in the US, Larry has a list for it.