I have two thing to contribute to Halloween food, but the second one will have its own pot, because it's long.
I didn't take a picture of these because they didn't quite turn out (aesthetically) the way I wanted. I found this recipe way back at 4th of July, and adapted the colors/flavors for the spooky holiday. See the original here.
So I won't type out the whole thing (you can go read it there), but I'll tell you my adaptation. I did a layer of yellow (lemon), then a layer of white (piña colada if you can find it, but there's a recipe in the link if you can't that involves sweetened condensed milk), then black (black cherry with extra food coloring to darken it), then orange (peach with extra coloring...I though I had orange, but I didn't), and then purple (grape). Unfortunately, you could tell the orange apart from the two dark colors on either side of it, so it looked like yellow-white-thick dark layer.
This recipe could easily be adapted for any colorful holiday -- Valentine's Day, St. Patrick's Day, 4th of July, Christmas, even Easter.
Be sure you read the recipe all the way through before starting, because you don't make the Jell-O the typical way. It's a quick set method.