I think a good 7th grade science project in physical science could be to study solar cells and how they work. Maybe power something with it, and perhaps give updates on the current technology (I recently heard that they reached new efficiencies with how much energy they could convert!). The easiest of course is to buy a cheap $5 calculator (or two) and take it apart for the solar cells. You might be able to find them in other places as well. Theres a bit about semi-conductors and the photo-electric effect involved, which are all solid pieces of science that you could investigate. Anyways, hope this helps!

Im not an earth/life scientist, so I don't know much about those, but perhaps for life science a study on the life cycle of a short-lived animal (mosquito, housefly, frog, etc). Or maybe for earth science on the decomposition of different materials in landfills. These are just suggestions, and I don't know what's practical for you.

If you want something that's mixed field, maybe the methods of classifying and decoding the human genome would be interesting (part database, computer science, and life science) to study on the surface. You can give demos using a computer you own yourself on how things are classified and stored, but I don't know if this qualifies. However, the future of lots of sciences depends on inter-field mixing, so it would be useful to learn, although less fundamental than a regular hard science.