Adventures of a Middle Sister: DIY Tie-Dye Swimsuit Cover-up. Plain white oversized T-shirt, tie-dye kit, scissors, needle & thread. Could use a T-shirt that's already colorful and skip the tie-dye step.
DIY tank top - item: scissors, bleach, plain tank top/t-shirt. Dip dye the bottom of the shirt with bleach. Wash it out and let it dry completely. Then cut the part you bleached into thin strips and tie a knot at the ends.