In the YouTube video embedded below, I show how you can build your own barometer out of some common household items. This is an easy project and everyone in my son's Cub Scout den built one of these.
The material list for this project is:
-plastic cup, glass, or can
-9" or 12" balloon
-wooden skewer or straw
-foam board or wood
-glue and tape
A glass jar or metal can will work better than a plastic cup, but the rigid plastic cup I used for this project works reasonably well. The balloon is cut and stretched over the top of the container, then a wooden skewer or straw is glued to the middle of the balloon. Build a simple scale to track changes and you are done. As air pressure changes, it will force the balloon membrane up and done, causing the pointer to shift up and down as well, and allowing you to track air pressure changes.
For best results, build the barometer when the air pressure is near average, and not while a strong high pressure or low pressure is located overhead.