"Laser pointers over about 100mW can pop balloons and melt plastic. Dark or black balloons are required because they are better at absorbing green light. White or light coloured balloons will have too much reflection and the absorbed energy will not be enough to pop the balloon."
http://www.ebay.co.uk/gds/Green-laser-pointers-/10000000008884174/g.html
Ahh. Perfect. Just a small amount of some sort of red oil. I could dress as Bob Gray and hand out balloons to kids then covertly pop them spraying "blood" everywhere. Brilliant.
http://www.ebay.co.uk/gds/Green-laser-pointers-/10000000008884174/g.html