Emergency Whistle Substitutes
I once got separated from my group and I did not have a whistle on me (because I was dumb), and I couldn't quite produce a loud enough whistle on my own. Luckily I had an acorn cap in my pocket, which I can whistle loud enough to get everyone to cover their ears!
You should learn how to whistle this way, because it works on lots of things and it could save your life! All you need is a small cavity. Bottle caps work great, too.
It's very easy to do, but before you've done it the first time you'll probably struggle a bit. Once you get it, you'll be able to whistle caps and break ears like a pro!
First, find yourself something shaped like an acorn cap or a bottle cap. Bigger is easier, within reason. I found a cap for a contact case which is very big, probably the maximum size you'd want to try for this.
I've marked the area you need to cover with your hands with a marker. Don't let any air escape from the black area!
Cover the cap with your hands as shown in the picture above. Make sure it's airtight. The only contact your hands should be making with the inside of the cap is around the ring. With your hands placed like this, place your bottom lip across the upper knuckles of your thumb and blow! You'll need to practice to find just the right way to hold it and the right way to blow, but you'll get it with practice!
Of course, you should have an actual whistle for camping.