To the Seaside!
Ideally, you'll need two swimsuits - one for swimming and one for sun-bathing. Don't bother to take sun-oil with you, the kind you find abroad is excellent and cheap.
Also, take a look at this old post about retro swimsuits.
Take a four-piece beach set :
Remember that most continental resorts object to tourists wandering about their streets in brief swimsuits, so take some sort of beach wrap or shift dress to slip over your swimsuit from hotel to beach.
Buy a sun-hat when you're there - they're much prettier on the continent.
You probably won't find it necessary to wear evening dress for dinner in the evening, but you should change into a light dress, so pack two or three. Everything dries very quickly in hot climates, so take two changes of cotton undies. Try to avoid taking stiff, frilly petticoats; they are very hot and prickly and take up far too much room.
Nobody seems to wear stockings on continental seaside holidays but I should take a pair just in case! One pair of sandals should be enough and you'll find that these are very cheap abroad. Take a pair of little flat pumps or court shoes to wear with dresses.
Try to avoid wearing cardigans in the evenings - they stamp you as an English tourist right away! If you do feel chilly, throw your cardigan around your shoulders or get a stole to match your dress
And to truly complete your beach wardrobe, how to make a poncho and beach bag (from 'Smart Girl' 1964) :