Gemmin is a light festival which consists on throwing a water-soluble glow-in-the-dark plastic gem into the flowing river. The movement of the water will create interesting visual patterns. This festival can also be gamified by putting messages inside the gem which will be collected by other participants. In return, you can also gather a gem from the river and see the message you got. Since the gem is soluble on water you need to get it before it (and the message inside) melts into the river.
To see how the light festival would look like check out my Kelvingrove Under The Lights illustrations.