Really loving this demonstration of fluid density from Victoria, Canada's School-Based Weather Station Network. It's incredibly simple, but surprisingly effective. Every grade school on Earth should have this setup.
This video has a fun and simple demonstration of the importance of fluid density in buoyancy and stratification. Fresh water (red) and salt water (blue) are released together into a small tank. Being lighter and less dense, the red water settles on top of the blue water, though some internal waves muddy their interface.
Watch what happen when half of the settled water is separated, mixed, and released again. Incredible how videos like these can be arresting even if you're already familiar with the science behind them. (On that note: here's an awesome fluid-dynamics video with science you're probably not familiar with.)