Make it water proof, make it practical. Here are my favs:
1. Lotion with sunscreen (australian gold 30) Australian Gold no aerosol spray. Hard lesson learned - since the spray covers erratically- streaky burns.
2. Mesh bags, waterproof, sand proof! Even Ikea has them. Use them for chapstick with sunscreen, small comb and mirror, bit of cash, and add coin purse for jewelry. I like to keep a supply of first aid /SOS stuffs like tissues, nail clipper, tiny swiss army knife, aspirin, excedrin, bandaids.
3. Sarong or turkish towels- use as a cover-up to and from the pool/beach AND, they make great sand towels, lightweight cover from too much sun- extends the sides of an umbrella with towel clips.