Why do we always forget things when we go on holiday? It is almost guaranteed that during your holiday you will realize you really need something which you thought about packing, but left out. I think it is the huge fees that airlines charge for every kilo you are over the allowed weight that make us forget things, as we are constantly trying to trim down the weight by taking things out and rearranging stuff.
Yet I think your foreign currency is one of the most important things you can pack, and that doesn’t weight much at all! I know you can easily change money once you get to the country, but then you have to find somewhere that is open, check that they are not giving you a bad rate and find out their fees. Get a chunk of cash changed before you leave so you get the best rate. Something else people tend to neglect as you can buy it once you are there, is sun cream, but again if you wait until you get there then you could find they are massively overcharging you as they have you cornered.
Something you can easily pack in your hand luggage, but might be a lifesaver, is a photocopy of your passport and other travel documents. Just in case you get mugged then place these copies in the hotel safe so you can get them if you need them. You really don’t want to be stuck in a foreign country without your passport and without your insurance details etc, so make sure you do this, and write down the number of who you need to call if your passport is stolen.
You need to pack a good camera too, like a waterproof one. You need to take some sort of camera with you, and you can get all sorts of waterproof ones now from Amazon, even a pink waterproof digital camera if you really want a colorful one. That way you can take pictures above and below water, without needing two different cameras of a bulky waterproof case etc.