Getting a big bowl of water or doing so in a small pool or bath tub would work as well. Different toys that the child likes, different objects that can be found outside, sticks, rocks, leaves, etc. Before putting the object in the water, make a hypothesis (educated guess) on if the object will float or sink.
Make a chart. Charts are an awesome way to document progress and conclusions. Draw a picture of the object, or take a picture and put it on the chart.
To document whether the object floated or sunk, draw an arrow pointing up or an arrow pointing down. This shows whether or not the object floated and will help the child make comparisons and observations.
Ask lots of "open ended" questions. These kinds of questions do not require a yes or no answer. The child can be as explanatory as they please or as much as they are encouraged.
Note: This would be fun with toy boats!
Use words like: Float, Sink, Up, Down, Heavy, Light, Hard, Soft, Big, Small, and Medium.