This teacher did this for a preschool lesson, but it can be adapted for older kids like my fifth graders. They could drop or open the balloons and observe what happens to the solid liquid and gas inside it. This would help identify the properties of solids, liquids and gases.
Also on these boards
Water Balloon Name Game~Fun Summer Learning Activity from Fantastic Fun and Learning
Water Shooting Painting--Fun, active process art for kids
Some beautiful ideas for outside art projects.