I am currently pursuing my Masters in Secondary Education with emphasis in Physics, but I student-teach Honors Physical Science. We are big on literacy, and introducing vocabulary terms to elementary students not only familiarizes them with science, but also prepares them for years to come. Terms frequently used in science like compare and contrast, cause and effect, analyze, etc. are great to incorporate early and have students understand fully so that they can appreciate the investigations and experimentation that comes in Secondary School. As far as experiments that can be done early on, I like to recommend the Law of Conservation of Mass with vinegar and baking soda in a zip lock bag. Most students can find these materials at home, and they may not know the underlying cause of what is happening, but at least they can see the reaction (and it is safe) and that when the two are mixed it causes something else to happen. This peaks their curiosity and has them wanting to know more and ask questions or dig deeper! Hope this helps!