Look at your detector, look at anything that can break that would stop you from detecting other then the controls dying.
I take a tackle box with me every time I hunt that contains: (Stays in my van at all times)
Spare AA (usually 24), AAA (usually 12), and 9 Volt batteries (usually 4) watch batteries for my head lamp (3 sets)
Spare Nylon bolts and nuts for attaching coil (currently have 4 sets)
Spare rubber washers for end of shaft (currently have 6)
Allen wrench
Multibit screw driver
Vise grip plyers
Hand full of 8 inch tie wraps
Hand full of 3 inch tie wraps
Tie wrap cutter, cuts tie wrap flush
Pocket knife
Roll of velcro
Roll of electrical tape
Spare shoulder harness for the detector (Stays in my van at all times)
Spare bungies for the harness (Stays in my van at all times)
Spare water bags for trash and finds (Stays in my van at all times)
Waterproof case for keys (Stays in my van at all times)
Spare scoop (Stays in my van at all times)
Spare water detectors in case one has a problem ( I never go with less then 2 most times 3 detectors, ask my partners...

)
Spare Rash Guard shirts
Wet suit
2 water hats with neck guards for sun. (hats are SPF50) (Stays in my van at all times)
2 sets of shoes for hunting in the water
Cooler that contains water, RC colas, and sandwiches........
First Aid Kit
Items that do not stay in my van are usually packed in a carry on bag and ready to go at a moments notice except for the wetsuit.
After driving 1.5 to 3.5 hours to get to the beach I am going to hunt, the last thing I want is for something to break that stops me from hunting.