mattnum,

Several bottles of aspirin would likely make a good candidate for the use you describe. It takes relatively little space, does not cost much in the generic brands, stores fairly well, addresses muscleaches, headaches, toothaches, may help out in certain heart related issues (see disclaimer below).

Having it in a variety of standard manufacturer package sizes would likely provide additional flexibility as it can be left factory sealed in the different sizes for bartering of different value items.

Disclaimer: NOTE: I'm not a doctor, nor do I profess to be, or even pretend to play one on TV. Always be sure to discuss your health and medical issues / concerns with your doctor or medical professional before using any medicine.

Regards,
Comanche7