Apart from all the usual bits like charger, beach towels, suncream, first aid kit, travel insurance and credit card, we concentrate a lot on DD's requirements.
She (16) has severe anxiety and we are waiting for the ASD diagnosis. The most important bits for her are:
several stress toys, at least one in each bag and also in the suitcases
at least one more set of headphones (in ear) in case the cable breaks as she has the habit of playing with them
whatever stuffed toys she currently favours
any other comfort item, in the past we took small pillows, fleece blanket, fluffy cardigan with us
her favourite way to relax and easy to transport are pads of paper and colouring pencils
Also, pool toys like snorkle, diving sticks, small balls. Take water shoes if there is the smalles chance the beach has pebbles.