What a great question! Science fair projects can be a blast to complete, and they teach kids valuable science concepts, too. Education.com has about 1500 science fair project ideas available on the site.
The portal below contains a tool that allows you to narrow the results by grade and topic - like physical science, earth science, life science, etc. The link to all of these projects is included in the Resources section below.
I'm also including a guide to Science Fair Project Do's and Don'ts, an article that your child may find helpful.
Hope your child finds an interesting and fun science fair project to do! Good luck!