The night before you want to try this activity, put a couple of toy plastic dinosaurs or sea creatures into a sandwich bag for each child, along with a few shells or pebbles. Fill the bags with water, then twist or tie them shut & pop them into the freezer.
The next day, peel the dinosaur icebergs out of their wrappers and put each one on a tray, along with with a few utensils - spoons, spatulas, maybe a turkey baster (nothing too sharp). Then challenge the children to free the 'fossils' as fast as possible.
Ask them a few questions as they work. How does the ice feel? Which tool works best? What happens if they put their iceberg in the sun, pour a cup of warm water over it, or add some salt?