I usually keep the following handy in my car. A knife, screwdrivers (slotted and phillips), electrical tape to tape the rigging, pliers, tape measure, extra split rings, cotter pins, pins for the blocks, extra cordage of various sizes, boat soap, sponge, brush, dust pan and broom and a 5 gallon bucket.

Rigging knife, wire cutters, Floatation cushion and bungee ( if you capsize, swim to mast to attach so it doesn't turtle ).  Extra line - to pull mast up if it comes down, and to be towed if necessary. Extra halyards and (1) extra stay.  Extra bolt/nuts for block : rudderhead attachment.  Towel, grease for winch assemb, zip lock bags, and a lantern to attach to mast if you get stuck in dark. An anchor with plenty of line. Flares for emergnecy.  Lots of rum and (6) paks of pineapple juice, cups, coozie's !  
I boat in a large, windy, choppy bay where weather and immediate assistance are variables.  If something goes wrong, I'm ready to deal with it.