I normally carry a small wrench in my pocket, because you just never know when you might need to work on something. The wrench that I have had for years is a 4" crescent or adjustable open end wrench. It is small and light weight, unfortunately it only opens up to 1/2" and many bolts have a 9/16" or larger head.
The green wrench is my old 4", the silver one is a new 6" crescent wrench that I modified. The 6" wrench will open much bigger than 1/2", but it was too big to carry in my pocket. I decided to cut it down shorter and add a screw driver to the handle.
The end result is just a little longer than the green wrench, but will still fit in my pocket ok. The screw driver part is from a "4 in 1" screw driver, it has a removable bit with philips on one end and a straight blade on the other. I cut the handle off the wrench and the screw driver, then welded the pieces together. After welding I ground and polished all the sharp corners off.