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Create a sprite or model of a famous artifact, and use it in your prod
The mask of Montezuma shown above is freely shared 3D model by theoxyz on Sketchfab and available for download under Creative Commons (CC) terms. The challenge is to use this or a similar real-world artifact for your real-time Graffathon production.
Montezuma's headdress is a well known example of a precious object of historical and political relevance. However, no freely accessible (CC- or PD-licensed) models can be easily found online. There are plenty of photos if you would like to create a replica.
You might also be able to find a scan of another object with an interesting story online. Some museums, such as Małopolska in Poland are uploading thousands of objects in accessible formats. See also Europeana.eu