Have you tried 'The Unfair "build your own crystal" race'? This is a crystal-building race which shows the greater the time available, the larger the crystals.
The activity can be used as a ‘starter’ or plenary activity for a lesson on the crystallisation of igneous rocks, emphasising that the more time there is available for the magma to cool and crystallise, the larger the crystals will be. A coarse-grained igneous rock (granite) may have taken thousands of years to cool; the medium-grained rock (microgranite) may have taken years or hundreds of years to crystallise
whilst the fine-grained rock (rhyolite) may have solidified in days or weeks.
Visit the website and the link to 'Teaching strategies' to find lots more ideas.