What kind of vehicle or item are you needing to wrench on? The possibilities are endless.
Personally, for the car I carry jumper cables, a lug wrench, and a multi-purpose tool. And a AAA card.
For my motorcycle, I carry a simple roll that has wrenches to match the various bolt sizes, allen wrenches to match as well, a screwdriver, multi-purpose tool, a tire gauge, a tire plug kit, and a small air compressor. Every time I work on the bike I use the kit and if discover that I need to add something, I get one for the kit. Being a Honda, the bike kit has never been needed while traveling.