If I had a pool I would totally try this out. Put a glow stick in a balloon for pool lanterns. Pool party on a Summer night! I think this could work pinned up on the fence of a backyard without a pool, too, so really great idea for any outdoor BBQ/party!
How to keep a child busy for hours in the summer: DIY: Giant Ice Cube Awesomeness - filled with plastic animals, dinos, and such. And I'm sure you could put in food dye before its frozen to color it like the ice block in the photo.
With chalk paint or even just water! Washable Chalk Paint for Kids. Peyton and Handyn did this today and they had fun for hours. My backyard, fence, concrete, windows, and even the dogs are very colorful. Recommend to anyone with young kiddos.