A large trash bag. In my years of Boy Scouts, I only learned two things:
1. Do not throw M-80's in the firestack for the Arrow of Light ceremony or you will be kicked out.
2. It rains in the woods.
A trash bag can double as a poncho, but I tend to use mine more to pack everything within my pack. It isn't 100% water-proof, but it does keep my gear dry-ish.
Also, check at Target stores when they have their $1 section set-up. I bought a ton of waterproof matches, emergency blankets, those nifty 5 in 1 camp whistles, and inflatable pillows and spent about $30. Never hurts to carry a spare in some instances.
While I am thinking about Target, Eddie Bauer has some of their heavy-duty Nalgene-esque bottles filled with some pretty handy FA-type gear. Space blankets, small FAK, whistle, flashlight, EB's version of a SAK, cyalume sticks, etc.
I grabbed two different models as I was prepping a PSK for my 10-yr. old brother. He didn't do BSA, chose Tae Kwon Do instead (for this, I am grateful) so his camping/survival/hiking comes from me and the other guys in the family. I am planning on taking him on a short day-hike when my family meets at this place called Potter's Creek for Labor Day.