I stick DL card key and truck key in my bathing suit pocket I wear under a 3mm tucked into the booties, nothing else necessary. Figuring most sane thieves wouldn't hassle with the small stuff, and not caring so much if they did, I've successfully left T-shirts and cheap sunglasses behind, often a few guilder coins in the ashtray, maybe a few bucks under the floormat for an ice cream on the way "home".
Save-a-dive kits aren't worth the risk, though bringing extra o-rings sounds like a good idea - I only dive (free!) nitrox on the island which means I've at least ensured that the tank seals against the pressure gauge and/or integrated analyzer before loading it, but it makes a lot of sense to bring extra o-rings or tanks on a more remote dive such as the Slaagbai sites. Probably tanks, since I have had the experience of a tank valve rolling open during a bumpy drive and dropping about 25% - no spare o-ring will get your air back, and that post-skip-breathing headache cost me a couple Advils and beers in order to get it properly cured. A tank rack can help with this, if you're lucky enough to score one, and I haven't yet heard of a case where someone stole a tank rack to chop up for firewood (though that theory could help explain why they're often scarce).
Another gadget you might want to bring with you, either underwater in a sealed case or artfully stashed in your dive vehicle, is a BC-to-tire inflator "thingy". I got one based on sage advice from either this board or that local board, but have yet to manage to bring it with me on a trip since the little bugger seems to get lost everytime I start packing only to rematerialize a week after I get home. Fortunately I haven't had a flat yet, but if I did suffer one that was re-inflatable (perhaps local thieves will graduate from siphoning gas to stealing tire air!) and found myself without my "thingy", as soon as I returned home I'd make sure to throw the damn thing deep in the nearest ocean for getting itself lost when I needed it most. (I admittedly have used the "thingy" at home several times when I procrastinated over getting nails removed from my own dive vehicle's tires, but it's a real pain having to lug the tank all the way through my house, dig out the regulator and BC, and suffer the embarrassment of having my neighbors watch me fill my tire using my scuba gear.)
Having recently returned from a wonderful dive trip and with no prospects of another any time soon, I am jealous. Have fun!