What about dive computers and other stuff? Is this something that you rent when there or something the guide has when doing guided tours?
When you are going guided, usually the guide will tell you the depth to keep. All very nice and don't worry about a thing... Until something goes wrong.
You can rent computers - 10-15 days diving pays for your own. I would suggest a simple wrist-mount computer as a fairly early purchase - this can be second hand, providing you can change the batter(y/ies) yourself. Check online for known problems and compare with other depth gauges to sanity check. Rental regulators usually have a pod with spg + depth gauge. Unless you need your hands free for spear fishing or working, wrist mount is convenient, as you can get it into view trivially to see depth, time elapsed, remaining NDL, deco obligation etc... I have found (both from myself and observing others) that knowing your depth/time without searching makes for calmer diving - the feeling of control is quite relaxing. Suunto makes good, if rather conservative, computers. Pay attention to how it lights up - the Mares M1/M2/Nemo do this well. Some need lighting, some a phosphorescent. Either self-lighting or phosphorescent, forget needing to shine a lamp on it to read in the dark (see next point). OLED is the best, but only for the high-end stuff at the moment and as a beginner / holiday diver, you don't need a Predator or X1.
If you are considering night diving, a lamp is well worth it. Doesn't need to be anything special, check DealExtreme. Second hand + rechargeable batteries = potential expensive battery replacement coming up. Consider getting a battery driven LED light such as the Schulz GS35 or GS45, or a similar cheap-ish LED. Yes, reds suck with LEDs. Halogens that give enough light are more expensive and last shorter. HID is nice and expensive. With batteries, just pull them before flying, the lamp will fit into carry-on luggage. Nothing sucks more than a dead light on a night dive, so having your own in addition to the supplied is the way forward. Lights tend to get banged about a bit, so either get cheap to replace or rugged enough to survive.
DSMBs are worth having (and practising with!), same as line cutters/knives. Some area dislike divers carrying knives due to a perceived threatening of innocent sea life, so a cheap line-cutter hook avoids discussions and can still get you untangled from lines.
If you are following a guide - which you will be initially - you don't need a compass. You will need one for AOW, though, so if you see a good deal on a Suunto SK7, get one. If you decide to get a compass, get a Suunto SK7. You want easy to read, robust, works at high tilt angles - in other words, an SK7.
More expensive computers have an electronic compass built in. Some like, others don't, price sensibly and choose yourself.
For the rest... don't worry, diving equipment tends to randomly accumulate.
If you are in the shop anyway, consider getting a spare mask band and either spare fin straps or spring straps, as well as a couple of spare regulator mouthpieces, a bag of cable ties and an o-ring kit. This stuff is useful and sometimes the dive centres like to charge you an arm and a leg for saving your dive, so you can charge only an arm and your fellow divers will love you.
Hope that helps.
Gerbs