I’m expert at this.
For girls:
A plain tank top or short-sleeve shirt with one of those high-waist skirts with flats. For accessories, a necklace with multiple chains or a long necklace with a not-so-fancy pendent, and matching earrings.
For guys:
A T-shirt with his favorite brand’s logo such as billabong or volcom stone, paired with neutral colored cargo shorts or colorful surf trunks. For footwear a pair of flip-flops or sneakers, and maybe a pair of shades to match.