This costume that I found on IKEA Hackers is brilliant!

Ingredients:

• 1 Regolit Shade Lamp from IKEA
• tape
• paint

Directions:

• Make the top hole bigger, so your head fits in it, but don’t cut near the metalic ‘loopies’; you still need them to keep the metal structure in place and stretch the lamp open.
• Reinforce with some tape.
• Cut a little spyhole.
• Paint a pupil & iris around it.
• Put the sphere on your head, then stretch it open by introducing the metal structure.
• And remember to bring a straw if you want to drink at the party!

What would be really cool is if you do this with a group of friends, so you all show up at the party with giant eyeball heads.