The link above will direct you to a great site listing tons of great science experiments that can be completed at home with common materials. One of my personal favorites is the naked egg experiment. All you need is vinegar and eggs! ​https://www.stevespanglerscience.com/lab/experiments/amazing-egg/
Make your own roller coaster using some pipe insulation from the hardware store and a marble!
These videos outline the process of making your own air powered stomp rocket launcher using PVC and a 2 liter soda bottle. They also outline basic paper rocket design. There are multiple videos here using different processes. Find the one that suits you best and get to building!