If blatant and utter cheating (picking out something nice at the caterer's) is out of the question, let your "preparation" be more assembly than actual cooking.
You already have a couple of suggestions that lend themselves to that. I will give you another one:
Faux Black Forest Cake
Buy an unfrosted devil's food cake, a can of cherry pie filling, some chocolate syrup, some whipped cream, and a bittersweet chocolate bar.
With a long knife, split the cake horizontally, so you have two "layers."
Drizzle some chocolate syrup on a plate, and set the first layer there.
Spread half the cherry pie filling over the top of the first layer. (If you bought a big cake, you might need two cans of cherry pie filling)
Carefully place the top layer on top of the cherry-pie-filling-covered bottom layer, and drizzle chocolate syrup on it.
Glop the rest of the cherry pie filling on it so it runs down the sides.
Squirt whipped cream over the whole thing.
Take a cheese grater, and hold it over the cake. Run the bittersweet chocolate bar over the cheese grater until cake is sprinkled attractively with chocolate bits.
An hour will be plenty of time for the cake to absorb the various things it is intended to absorb.
You can also do a variation on this using angel food cake, crushed pineapple, whipped cream, and sliced toasted almonds. Or you can use mandarin orange slices either instead of or in addition to the pineapple, coconut with the almonds, discover explore. If you like caramel, do oranges, caramel sauce and pecans. Or walnuts.
You can see this is very flexible, and gives you plenty of room for creativity. The key principle is assemble - don't cook! Astonish! Amaze! Deceive! Confound!
