You also could just make a regular spice cake, using either a mix or a recipe, even a carrot cake would be good. Or, you could make a yellow cake and stir in Halloween sprinkles, i.e. a confetti cake.
I've decorated many cakes over the years, but the ones that went over the best were some I made for 1 of my daughter's parties in her teen years, cupcakes. They were decorated in several different ways.
Pumpkin patch, using the little pumpkins made out of the candy corn stuff, green frosting grass using the hair/grass decorating tip.
Pumpkin, orange frosting, candy corn to make the eyes, nose, and teeth, black gel icing to outline the mouth.
Ghosts, dark blue or colored black chocolate frosting. Ghosts made out of melted white chocolate and cut out while cool enough to form shape, but not cool enough to crack while cutting. I stood the ghosts upright.
Night sky, Dark blue frosting, moon out of melted white chocolate, stars out of slightly tinted yellow white chocolate, though I might use sugar or isomalt for the stars now. I'd definitely use some luster dust for the stars though regardless.
Black cat, green grass frosting, chocolate cat or decorated cookie with an arched back standing up. You could also throw a pumpkin or 2 depending on size of cat you make, or maybe some bone shapes either piped or cut out of chocolate. Maybe a tombstone.
Spider, green grass frosting. I piped a body, added red eyes, and piped chocolate for legs so they'd stand up. You could also use the Royal icing for this, though now I'd probably use melted, dyed black sugar or isomalt for the legs now, or maybe modeling chocolate.
I did a witch on a broom with a moon too, but I don't remember whether I used a template to pipe or cut her out of chocolate, but either would work.
You could also use Royal icing instead of chocolate for any of the ones I mentioned using chocolate.
They make a lot more candies now that could be used for decorating than they did when DD was a teen, eyeballs, bugs, etc.