A classic…
Fill a small aquarium (or other clear container that can hold enough water) with water. Place a can of Coke and a can of Diet Coke in the water. The Diet Coke will float while the regular Coke sinks. Very little artificial sweetner is needed to sweeten the Diet Coke, as compared to the sugar used in regular Coke.
Some extensions…
Test your students understanding of density by using different sized cans of soda, if you can find them.
Measure out the amount of sugar and artificial sweetner used in each can to show students. Once you have it measured, store them in small plastic bags, so you have them for next time.