Go to party city. They have gobs of costumes and masks. You're sure to find something there you both agree on.

Could you try some kind of skeleton? There are some cute ones that aren't too scary. My younger DC wanted a bloody mask after walking around costume shop but we comprimsed on a skeleton mask with no blood.

I said that "babies" (younger kids at preschool/daycare) who saw one with blood would cry.