Liquids interact with each other in many ways. If you were to do a science project about different types of liquids, it would make an interesting experiment. Think of vinegar and oil. If you pour them into the same glass what will happen? How about whiskey and water. If you pour them in a glass together what will happen?
You can test the density of liquids. Which is easy to do with simple buoyancy tests. Take a set of three or five liquids. Get them poured into similar containers and in equal measures. Now collect a few light and heavy things that you can check buoyancy with. If you record your observations in a systematic manner, your science fair project is done.
All you need to do is get a basic principle and test it. That’s an instant science project for you. Take Surface Tension for example. Is it the same for all fluids? No it isn’t. How do you prove it? That can become a good science fair project if you delve into it.
It is not possible to talk about fluids and not mention bubbles. You know its great fun blowing bubbles with soapy water. But do you know the kind of soap you mix in the water affects the kind of bubbles you will be able to blow. Check it out some time. Here’s one simple experiment of interacting liquids that you might enjoy.