Glow Sticks. This is a seriously cool and economical way to take your parties and decor to a whole new level.
Glow Pack for costuming and decor
Glow sticks inside balloons, some cheesecloth, and glue-on black mouth, eyes, and nose
I did this for a postapocalyptic-themed Halloween party. Green lens on the pool light, giant spiders made with trash bags on the walls around the pool, and empty water jugs with glow stick green inside, a hazard waste sticker, bobbing on the pool.
Take it to a creative level!