Kool-aid powder and/or food colouring
Some cold weather or a freezer
Put a few drops of food colouring or Kool-aid powder into a balloon then fill with water.
Place outside, if it's cold enough, or in the freezer. Depending on how big the balloons are and how cold it is, this will take about 24 hours. Hold them up to the light to see if they are frozen all the way through.
Use a knife to cut the balloon off the frozen ice marble.
Food colouring makes more of a marble look (the blue one in the picture), Kool-aid makes a solid colour all the way through.