First, I made Bigfoot hunter t-shirts and ID badges for all of the invitees.
The boys had to earn their badges and equipment bags (nets, toy binoculars, flashlights) by beating up the Bigfoot pinata. I'm actually pretty proud of this pinata -- it's a monkey we found at Walmart, but we amputated his tail and gave him "angry" eyes and mouth to make him a Bigfoot.
So once all of the boys got their badges and equipment, we took off into the woods to look for clues to prove Bigfoot's existence. And BOY did we find them! The boys founds multiple Bigfoot footprints, claw marks, and bigfoot hair in the woods. Unmistakable proof that the big guy roams the area. Luckily for us, it was daytime, and everyone knows that Bigfoot doesn't come out during the day!