The ‘Ol Pregnant Turkey Prank
Apparently this is a somewhat popular prank... although I've never heard of it. It seems pretty simple (and clever) too. You just take a Cornish game hen, which looks like a little chicken or turkey, and stick it inside your turkey before you cook it. Then you have your victim start to prepare the bird for consumption. Btw, it helps if you're victim isn't the brightest bulb in the drawer. Now in theory, once your victim sees the game hen inside, they will think the turkey was pregnant, and y'all just killed two birds.
Believe it or not, more than one person has fallen for this prank. It might be too late for you to try it this year...but there is always next Thanksgiving.