I am an Elementary Education student at Penn State University. Over the past few summers, I have worked with kids who participated in online school and virtual/at home camps. Some examples of engaging hands on activities we did together included growing and monitoring seeds, and baking soda and vinegar chemical reactions. These activities are hands on and are super interesting, and only require materials that can be easily found in a kitchen already or requires only a quick trip to the dollar store or grocery market. Adult supervision is required, and assistance may be needed, but that will come with any experiment when working with elementary students. Constructing take home kits with the materials kids will need is a great way to engage them in tactile experiments while teaching virtually. Once experiments have been conducted, students can reconvene online and discuss their findings, and if you use any sort of online school platform, you can have them complete reflections on their experiments online as well. To answer your second question, an at home science fair could work by having students design or choose an experiment of their own and then doing a "gallery walk" one day during online school. Students, with the assistance of an adult, would be able to show the class what they completed in their experiment using photos, videos, or even a live demonstration. Hope this helps!