I liked Lyle Sadavoy's ideas about the Elephant's Toothpaste demo (Chem 13 News, September 2016).
He suggested placing the cylinder in a basin and covering the workspace with paper. I suggest placing white paper under the cylinder (or conical flask), and placing that in the basin or on a tray. When the foam overflows on to the paper, the paper turns blue. I would give a bonus to the first student who asks why the paper turned blue. They may not be ready for the answer (iodine-starch reaction), but the fact that they asked the question entitles them to a bonus.