In the past I folded up a 1 sq. ft. section of aluminum foil and put it in my pocket. Negligible weight and size, and oh so many options. For instance, in a REAL jam, if you drop it in a bottle of toilet bowl cleaner, it... well, allow me to illustrate with youtube:http://www.youtube.com/v/tXsNHROx0oU&hl=en&fs=1&
As a result, you would end up with caustic hydrochloric acid all over the place- especially if it got in eyes someone would be in a world of hurt. More effective as a diversion though I'm sure.