Split the group in to random teams and give each team a differnet colour string or yarn and a bunch of posicle sticks/skewers. Give them the following instruction: Create the best piece of lawn art by pushing popsicle sticks/skewers in to the ground and winding string between and around them, you may not cut the string. Set a reasonable time limit.

Scoring is, as always, completely arbitrary, more so than usual here, since you will be deciding who to award points to based on something we all know is in the eye of the beholder.

You can set any parameters you like if "lawn art" is too vague, "make the best animal" for example.

You could get someone else to judge it, just see who happens to be walking by.