In first grade, we focused on developing hypotheses and using guided inquiry to perform experiments, using the scientific method. Sink or Float is a very effective and easy experiment to do, helping the students to develop hypotheses and test them. This is the simple chart we used that the students filled out. Items one could use include lemon, lime, coconut, rock, etc.
A very effective you tube movie to demonstrate why things float is https://www.youtube.com/watch?v=y0SnFCs9z1g