You've probably played a game similar to this, but this is a great activity that can start Thanksgiving morning and last all day. If you're not in the US, the game is the same -- there's nothing that makes it Thanksgiving-specific.
In the morning, place a poster somewhere that is accessible. Draw one line on it and invite others to add one line at a time. Do not give any other instructions. You may not talk about what you are drawing at any time during the day. You may only add 1 contiguous line at a time. You may not add another line until someone else has added a line. The more people that participate, the more fun this is.
If you join in the fun, send me a pic. I'll post it. I promise!
Check out the creation at Journey into Unschooling if you need some inspiration -- they colored theirs!!