I usually carry:
a small hand towel (PakTowel)
several empty ziploc bags (varying sizes) and a small piece of tin foil
Ibuprofen, Immodium, and a few band aids
camera with extra battery
video camera with extra battery and DVD's
tix, id, and credit card in lanyard in my front pocket
small tube of hand sanitizer, small resealable wet ones wipes
sunglasses, chap stick
ponchos
two frozen bottles of water