You should always bring along spare parts and some basic tools on every diving excursion. Dive shops all sell Save-a-Dive kits that are a good start on this. Here is a list of some things you might want to carry in your spares kit:
o-rings for your tank valves and regs, and anything else that uses them
wire ties, large and small, lots of 'em
spare straps & buckles for mask, fins, snorkel, and knife
spare regulator mouthpiece & wire tie
a few feet of wreck reel line and surgical tubing
duct tape ( squeeze a small roll flat to save space )
silicone grease or O2 lube
extra batteries and bulbs for all your lights, computers, and camera
spare lanyards and brass snaps
stainless steel dive tool with wrenches, allens, and screw drivers ( pictured above )
stainless steel folding multi-tool with pliers, blades, and wire cutter
glues and sealants - neoprene cement, AquaSeal, Crazy Glue
bicycle tire patch kit - for small suit repairs
soap, powder and wax for drysuit if necessary
extra cam band - plastic buckles can break, especially in the cold