Float It!
One simple hands-on experiment is floating a potato.
A more quantitative, measurement-oriented version is using a hydrometer to test and measure salinity. Advanced classes can make various solutions of known density (by carefully measuring weight and volume), test with the hydrometer, then mix solutions of different concentrations and predict, then test, what densities result.
Connections can be made to the engineering of boat design - Plimsoll marks on the sides showing load lines, with variations for different densities of water (warm vs. cold, fresh vs. salt) - now you can understand the underlying concepts and importance.