I got this witch's caldron from JoAnn's, on sale, for $1.49. It's original use is for candy on Halloween night but I needed to fill it up until then. (Let's face it, if it was full of candy it wouldn't stay filled until Halloween!)
So what looks like witch's brew? Ripped up batting! I always have batting leftover from quilts that I keep around for such occasions. I got my daughter involved too, and she loved it. I'll bet you could use batting for spider's webs, too!
Anyway, let your imagination run wild with options.