My sister in law did this a few weeks ago, so I wanted to try it. It was super cheap and easy to throw together.
Mix 1 part water, 1 part cornstarch, and a little food coloring, and there you have it.
I only made a tiny bit, because I knew it would be spilled all over in a matter of minutes. This amount was 1 Tablespoon cornstarch and water.
She totally loved it!
It entertained her for at least a half an hour, and in toddler time, that's ages.
It cleans up really well too. Washes off easily. Easy peasy summer activity.