Some two part epoxy, a stick of Plumber's Mate (industrial strength plasticine), a soldering iron (not the crappy gas-powered ones), a small multimeter, some inner tube and sheet neoprene (solid not foam), jubilee clips, a spare KISS valve, a couple of batteries, a sensor and molex plug, assorted Swagelok bits and a full set of o-rings. Also in my KISS bag is a spoon (in case I overfill the scrubber... obviously never happens
), a couple of small wrenches and one biggie, a pin spanner, a few screwdrivers, a set of hex keys, Richard (the KISS bolt tool), a couple of metres of LP hose, some 1/8" NPT hose tails, a spare capped 1st stage for O2, a spare uncapped 1st stage for dil and an IP gauge. It all fits into a small bag, not much bigger than a paperback book. I picked up an imitation Leatherman with a socket/driver set for £5 (about $3). It's not amazing quality but it's got most tools you'd need.
I also usually take enough regulator bits so that I can go back to a full OC set up if I have to. I've already got the backplate and wing on the KISS so it's easy enough to swap back to OC. I just find it easier to carry enough regs to do the same dives OC than a massive spares kit and I'd also rather dive OC than on a Rebreather that I can't trust.