Boy, I wish I had a picture of this--but someone stole my camera during a Halloween party and didn't leave the film behind (which would have been kinda nice--but they were party crashers in costumes). Anyways, this is one of my favorites. I used this for my son's school parties as well, but it's fun for an adult Halloween party. It's a favorite and we usually use the camcorder to "film the autopsy," giving the cutter rubber gloves and a mask.
Bake a Red Velvet cake (basically, a bottle of red dye in a chocolate cake mix). You bake this in two pans: one round 8 or 9" pan and a large sheet cake pan. When they're done, cool and invert. I usually do this onto flour dusted tin foil on the counter. Take a knife and cut the sheet cake into thirds. One third will be the body rectangle. One third will be cut into two long strips for arms, one third will be cut into two long strips for legs. The round will be cut to an alien head shape--with a pointed chin and round top. Take a piece of dental floss and use it to slice these pieces into two (layers). Saw it back and forth and then set aside the top layer.
Now, here's the fun and creative part. You can use strawberry jam, but it's more fun to use things like white frosting with a medium green food coloring for the frosting between the layers. Spread that on one layer of each piece. Next, gummy worms and chow mein crispy noodles are fantastic for insides, sprinkle those over the frosting. If you want, you can use other candies to create organs/heart/lungs. This is for your own creative part, but some ideas include marshmallows that have been stretched and food dyed, a Baby Ruth bar, red vines... Put the layer on top of each piece and now assemble on a piece of long cardboard covered in a piece of tin foil like a shiny autopsy table. Once he's in place, frost him with a gray frosting (mix a bit of blue into the white frosting with a one drop of red and two drops of yellow)--adjust as necessary. You could also use fondant dyed gray if you are really good with cakes. Most folks will find the grayish frosting works well. Spread him nice and smooth and use a thin piece of clipped licorice for his slit mouth and two tiny cut ends of it for his nostril slits. For his eyes, you can use any number of things. I ended up using licorice cut into tiny strips and used to fan out into a circle, but be creative.
People usually fight over what piece of the alien they want to eat, but the really fun part is just simply filming the dramatic autopsy with a scalpel/gloves/mask.