Me and my band had our first gig recently and I was missing some particular things such as a drum key and a tuner. So this is my solution.
Step 1: Find a Good Sized Carabiner
you can find these at dicks sporting goods but some companies give them out for free.
Step 2: Add a Drum Key
for drummers you ABSOLUTLY HAVE TO HAVE A DRUM KEY for gigs. So find a ring of some sort or a ring from a water bottle. Then take some string and tie the ring to the key.
Step 3: Laser Pointer
It's not really good for anything but it's just fun to have. You can get these for 3 or 4 dollars at a convenience store.
Step 4: Tuner
This is acctually a pitch pipe and if you can't match pitch then your done with your keychain. But I took a pen and blew on it like a coke bottle and filed it down until it was an "E" witch was about 1/3 of the way down a full clear tube is an "A" and the full white tube is an "F".