Talking of buying stuff to leave behind - it's cheaper to buy a Tesco Value or Asda Smartprice iron/kettle/hairdryer/toaster than it is to hire them from the hotel. You can always pass them on to new arrivals on your last day for a couple of euros - ice cream money for the kids
As other people have said, I always take lilos from the £1 shop and leave them behind, also cheap swimming goggles, snorkel sets, frisbees etc.
I haven't done self-catering for a few years, but when I did, I took packets of pasta-in-sauce and savoury rice and also breakfast cereals. I didn't bother taking tinned stuff though, it's heavy in the suitcase
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I used to make sandwiches for lunch and take them down to the pool with some cans of pop from the fridge and some lovely fresh local fruit from the supermarket. That way you don't mind the expense of eating out in the evening. Keep your packed lunch cool by putting a couple of bottles of frozen water in the bag - and you can also drink the icy water in the afternoon when the sun gets really hot!
If you have to have your Cadburys chocolate, then pop a stash in your suitcase. Our brands of chocolate are very expensive abroad. I remember paying 2€ for a Creme Egg in Fuerteventura last year, just because I "needed" one