You could do a pumpkin or orange very easily.
Get an orange or white onesie and leggings or tights (you can skip the leggings if you will be inside) then get the same color of tulle, and tie it into a tutu. (You can use an elastic, strip of fabric, whatever to make it with. Ther are tons of no-sew tutorials online) then use a sharpie to draw a cute ghost face, or a jack o lantern face. Plus, it can be worn all month without being too costumey. :)
You can do the same thing to make just about anything with a few extras... Use black, add a tail and ears, and draw whiskers with eyeliner and you have a kitten. Use yellow and black, add wings, and you have a bumblebee. Brown onesie, yellow and orange tulle, and a floppy hat for a scarecrow.