Slippers.

Also put some ice on your water bottle; it’ll keep your water cool.

Trashbag, maybe not for what you bring but for the bits of crap you’ll find scattered around the beach

Bring a diaper (nappy). When you go swimming, you can put valuables in the diaper (cellphone, jewelry, etc) and not worry that anyone will take it.

• beer (cheaper away from the beach)
• hat for shade
• fishing pole if you like fishing
• cheap old phone if you have one, can swap your sim card to it. Keeps sand and water away from nice newer phone

I’m not going without a book/ereader.

And multiple cameras.

(Edit) particularly for the reader, I should mention “be careful of sand” and maybe have one of those little duster cans around to make sure the ports are clear. But it’s too much of what I value in between soaking in the water to not bring.)