Outfits for each day, plus a few spare (especially for baby).
Shoes, socks if necessary, undies.
Bring one cardi each in case it gets cool later on and a few old tee-shirts as cover-ups on the beach (bring travel wash and you can halve the amount of clothes you bring!)
Sunhats
Baby gear - nappies, wipes, cream, bottles if you use them, sterlising stuff if you use that, sippy cup etc.
Swimming things - costumes, pool toys, armbands, goggles if they use them etc
Toiletries - toothpaste, toothbrush, hairbrush and any products you might use, mini shampoo and conditioner, shower gel, sunscreen and aftersun if you use it.
Mini first aid kit? Insect repellant? You prob won't need it, to be honest. My DS has hayfever, so I'd bring his medication.
Toys/games/activities that are easy to pack. I usually pack these in a separate plane bag, along with babywipes, a spare set of clothes each and baby stuff like nappies.