Hypothesis is what you think. There is nothing as a good or bad hypothesis.
Have you done any research on what is density and what makes something float or sink? Density is mass divided by volume. If the density of something is lower than the density of the liquid, it would float in the liquid. So what do you think weighs light? (My hypothesis would the aluminum soda can, cause aluminum is quite light)
The density of water is 1 g/mL. If you add salt, it'll change the density of the water and how much it is changed depends on how much salt you use. So actually, rather of testing of so many things, I'd rather test how much salt added would change whether ONE object float or sink.
But if you want to test so many objects, it's your choice.