1. Tunnel boring machine balloon
    In real life, a tunnel boring machine is 300 feet long and 27 feet in diameter. The balloon could be close to actual size (but the weight would be a problem: 1,500 tons). http://twitter.com/mackenzieTBM
  2. Wally Waterdrop
  3. Toilet bowl balloon
    Raising awareness of sanitation and clean water.
  4. Giant microscope balloon
  5. Toilet paper roll
    Suggested by @natahathan