Balloon plant (Asclepias physocarpa) is an invasive weed, introduced to Hawaii as a fiber crop. A member of the milkweed family, it is considered to have medicinal properties, but parts of the plant are also poisonous.
I saw several of these plants in a pasture occupied by livestock and, like the cow in the photo, they were avoiding the plants. Consequently, the pasture consisted of close-cropped grass and a generous sprinkling of these scrubby plants, 2- to 3-feet high, though they can grow to 6-feet tall.
The ‘balloons’ are actually the fruits of the plants. When ripe, they’ll burst and release a multitude of white silky-haired seeds.