Are you doing Bobbing For Apples? I don't know why I think of that as a Halloween activity (in the Autumn I guess), but I do. You could even cut a hole in the bottom of a garbage bag to wear over their clothes - keeps (some of) the water out and their arms in. If it's a black garbage bag, they could even change out of their tops into the bag and not get completely soaked.
I also think of goldfish tosses as being a Halloween party activity, but I'm not sure the other parents would appreciate you sending their kids home with pets, however low maintenance.
Have you made ghosts with cheesecloth, glue/paste, and balloons? You soak squares of cheese cloth in paste then drape them over balloons, hang or prop them up so the cheesecloth drapes and let it dry. Draw faces on with a sharpie and then pop the balloons and you've got ghosts! This would be something to do ahead of time, not an activity, but would still be fun to do with your DSDs.
You can also make witches' heads out of apples - but you need to start now: peel an apple but don't remove the stem! Carve out eye sockets and a mouth and then tie a string to the stem and hang the apple somewhere less-humid. The apple will shrivle as it dries, making it look wrinkled and hag-like. You can add teeth with rice - poke holes with a toothpick and insert the rice. I'd wait a week or two before doing this so you know how the mouth is shaping up - otherwise you get that lamp-fish from Finding Nemo. Put raisins/small dried fruit in the eye sockets as they shrink and then glue some yarn on as hair. If you're feeling ambitious you can make a witch hat out of black construction paper and a dress from a black paper napkin or scrap fabric.
If you find a paper skelleton you could play pin the brain/heart/lungs/whatever-to-the-skelleton. You could even have the girls draw their own internal organs or make up a bunch of different ones and then see who can get it closest to where it belongs
Um...You don't by chance need other adults to help you run this party, do you? I have out-of-town guests that weekend but I don't
really like them...