Split the sandwich thins into 2 halves and place 6 on each of 2 baking sheets. Spoon 1 to 2 tablespoons pizza sauce over each sandwich thin, spreading it to the edges. Add the toppings to create each of the monsters.
For mummies: Tear the string cheese into thin strips. Arrange the strips horizontally across three pizzas like layers of a mummy. Nestle in two olive slices for eyes.
For jack-o'-lanterns: Cut out rounds from the Cheddar slices with a jar lid or 3 1/2-inch cookie cutter. Place the cut rounds on 3 pizzas. Trim pepperoni slices into triangles (3 per pizza; 2 for the eyes and 1 for the nose) and place on the Cheddar rounds. Cut an olive slice in half and use for a smile.
For ghosts: Trim the provolone slices along a bottom edge in a zig-zag pattern with kitchen shears. Place 2 black olive slices on each pizza to create the eyes.
For spiders: Sprinkle 2 to 3 tablespoons shredded mozzarella over 3 pizzas.
While the pizza is baking, cut 2 whole black olives in half for the bodies and cut the remaining olive slices in half for the legs. Assemble the spiders on the cheese once the pizzas come out of the oven and are still hot by inverting an olive half on each for a body, then arranging the legs around the bodies.
Bake all the pizzas until the cheese begins to melt, but before it bubbles, 8 to 10 minutes. (Browning or bubbling may ruin your shapes.)