I would say for a week in a hotel, my packing would be:
2 x something to swim in - swimsuit/tankini/bikini
2x shorts
3 x t-shirts/vests (to be worn with shorts for either sightseeing and breakfast, or just throwing on over your swimwear if you want to look more covered up for lunch)
2-3 smarter evening outfits - so summer dresses, maxi dresses, skirt and tops. (not super posh, but just something other than sightseeing clothes for dinner)
1 pair of flip flops for the beach
1 pair of sparkly/smarter sandals for evening
1 pair of 'sight seeing' footwear - so trainers or comfy sandals you can walk a distance in - these would be my 'travelling footwear' rather than packed.
a beach bag
sun hat
beach cover up of some sort (kaftan or wrap)
a lightwieght cardie or pashmina (we have been caught out on European holidays when it's been a bit cooler than I thought in the evenings and I've wanted to sit out on the balcony once DCs were asleep)
a small clutch/big purse. (You don't want to feel you have to take your beach bag to dinner, or a big handbag, but depending on where your room is in relation to the restaurant, you might want to take a few bits with you).
oh and if you have long hair that normally has to be styled, I can never be arsed on holiday when it's hot, and by the end of a week with the beach/sea/pool, it's a mess, so pack hair pins/grips so you can put your hair up easily for the evenings.
Sainsburys and Tescos both have some good cheap summer basics in if you don't already have a summer wardrobe to get most of this from.
(I'd also suggest if you do paint your toenails, make it not too dark a colour as beaches can chip the colour, and so a lighter colour it's less obvious if you've done "running repairs" before dinner - oh and pack the nail polish!)