Hat, suncream, sunsuit (ideally with long sleeves and legs - dunno why most have short sleeves!).
Swimming nappies for pool/sea - I found the ones in the canaries weren't a patch on the ones we use here.
A sun tent is extremely useful if you can get one into your luggage.
Snacks - for the plane (distraction!) and so you have some favourites handy, although you can get baby food and loads of easy snack food in the canaries it is handy having a supply of mini rice cakes, raisins etc
Canaries can be a little chilly at night so we have taken various different sleep options - short vests, long vest, sleepsuits.
Towels are cheap to buy out there if you need to save suitcase space.
Muslins, you may not use them at home any more but large ones are brilliant for shade over little legs in a pushchair, for mopping up food debris etc. Washable plastic bibs are brilliant too so you only need one for the whole week.
Don't forget the sippy cup and kids cutlery if your little one is using cutlery - it's a pain trying to feed them with an adult sized fork!
Hope that's helps, I'll add more if I think of it.