You will need:
Oil is hydrophobic, which means it doesn’t mix with water. It’s also less dense, which explains why it floats on top of the coloured water.
The Alka-Seltzer tablet contains two important chemicals acetyl-salicylic acid and bi-carb soda, a base. They don’t react while the tablet is dry, because they’re both solids, and can’t mix together. Once the tablet is dropped in the water however, these chemicals dissolve and mix together. When an acid and a base react, they produce lots of Carbon Dioxide (CO2). These bubbles of CO2 rise up through the liquids and push the water up into the oil, bringing the food colour with it. Once the bubbles of gas pop, the food coloured water then drop down to due it having a higher density than the oil... only to be lifted back up by more gas coming out of the Alk-Seltzer tablet!
See how the drop of water rise and fall? Blue-green algae cells are able to control their buoyancy, floating or sinking when they choose to. Inside the algal cells are vacuoles, tiny compartments that can hold gas. When the algae needs sunlight, it fills these vacuoles and floats to the surface. Then, it empties them, and sinks to the cooler water at the bottom of the river to find nutrients and grow. This allows blue-green algae to out-compete other algae in the water column, often resulting in dense & toxic algal blooms in our waterways.
Similar experiment: Dancing sultanas
Lava lamp version 2
Because the oil and water don't mix, you get some nice oily blobs circling around the bottle as you turn it upside down. Water is denser than oil and if left will settle to the bottom of the container.
Adding salt into the floating oil causes it to sink as it makes it more dense. However, once the oil reaches the bottom of the container the salt dissolves in the surrounding water, allowing the oil to rise back up to the top again.
Commercial lava lamps rely on heat from a lamp to expand oil, making it rise to the top of the lava lamp. This oil then eventually cools, contracting and falling down to the heat source to start the process again. This is known as a convection current which is very important in weather systems and ocean currents. Here is an experiment on establishing a convection current.