Duct Tape....because you can fix so many things with duct tape.

Spare mask.....just because I have one and it fits in the kit. It is probablly overkill, but then again you never know. For example this weekend we were on a dive and one of the other divers was ready to splash when her mask strap broke. Considering it would be a pain in the rear end for her to get out of her gear (double clyinders, dry suit and a lot of other heavy gear) to mess with a mask strap for 5 minutes, it was MUCH easier for her to reach into her bag and pull out a spare mask. Again, it is much different where you dive, when all you would do is take off your bc and you could fix the mask.