A good sound guy is a jack of all trades. On smaller gigs he (or she) may be the only real tech on site and as such be looked to for a lot more than just sound. That could be big things like keeping the event running on time, managing power distribution, or even helping out back stage getting actors ready.
Being MacGuyver is part of the job description. At the very least you're expected to have extra cables and adapters on hand. Gaff tape, batteries, Sharpies, Band-aids, the list goes on and on. We're the Boy Scouts of the entertainment industry and it's up to us to always be prepared.
I've always carried a little sewing kit with me. There's nothing like an ill timed rip in one's trousers to make you have a bad night. Often some gaff tape or a safety pin will do the trick so it's not often that I've had to fish the kit out of my glove box or jump bag, but it's good to know that it's there. Except when it wasn't.
The one time I really and truly needed a sewing kit I was miles from either of the ones I usually have close to hand. I was in a rented tux, in a limo, on the way to a wedding ceremony. When you're the best man it's pretty important to show up game ready. It meant a quick stop at a convenience store and it allowed the entire wedding party to stock up on Gatorade and breath mints. But the incident stuck with me and I decided to come up with a sewing kit I could keep in my wallet.
The kits you get at a gas station will do the trick but they're a little bulky. For some reason there's always a completely useless pair of microscopic scissors, a thimble and a measuring tape in there. I figured at its most basic you could just buy a small pack of needles and wrap some thread around it. But I dove a little deeper and finally came up with a solution that will let you keep a few needles, as much thread as you care to wind and even a few buttons right in your wallet without getting snagged on anythng. Here's a link to the page on instructables.com.
While we're at it. MacGuyver always seemed to have a mashed flat roll of duct tape in his bomber jacket. Here's a little bit smoother way to make sure you've always got a few feet of gaff and electrical tape on hand. It fits in your wallet too if you're careful.