Insect repellant!
I always bought my nappies on holiday- just the local brand but I took swim nappies (enough for two or three a day) - they were much more expensive. Also I found that not many places sell nappy bags.
Pack a plug in night light.
I'd take a swim ring. I hate arm bands! Even with a float suit, you still have to be guiding them the whole time because if they tip forward they might not be able to right themselves. A swim ring will allow you more freedom.
If your LO is sharing your room, it might help to bring a headtorch if you have one so you can do nappy changes etc... without turning the main light on and disturbing your sleeping OH!
For first-aid I'd take plasters, calpol, piriton, sudocrem and a thermometer.
Good toys to take:
A set of nesting eggs or stacking cups- can be used for playing hide and seek with snacks!
The humble balloon! I always pack a few, and you have a game of catch whilst waiting.
Finger Puppets
ELC star links
Fisher Price snap lock caterpillar
John Lewis discovery cube
Sticklebricks
Edushape curiosity cube
Check what the hotel will provide in the way of baby stuff e.g. bed rails, bath mats etc... and whether you have to reserve any of it in advance.
Don't forget bibs and beakers and a few plastic spoons.
Take lollies for the plane for your DS to suck during take off and descent to help the ears.