Sure have!
Recently I was babysitting for my young cousins, and they like to play this fun little game. You fold a piece of paper into thirds (since there are 3 of them) and you start off drawing the head. After you draw the head, you draw the neck, but you leave enough of it showing so that the next person can see it (but the head is folded over, so they can't see)
Once everyone is done, they pass it on, and the next person draws the body, and leaves space for the waist/legs. Then you pass it once again and finish off with the legs.
So I was thinking we create some sort of a template where the head and body and legs should meet up, have people choose if they want to do legs, body, or head, and go nuts!
That's just one of them, I don't have much time for a longer reply. I thought that would be fun for a multiple person one.
A more simple, one-person activity would be to have a smaller canvas with a scribble on it, and you would have to somehow incorporate it into your drawing.
I'd also like to reincorporate the learning activity things, like the dithering and AA lessons. It's great that lief spearheaded that initiative, but it'd also be great to have a more structured, official one (no offense to ya lief)
So that's just my idea for the challenges/activites. I've got more, but once again, not a whole lot of time.