This is a wonderful science education site from India. There are literally several hundred easy little toys to make from paper and other scraps. He does not go deeply into the science of each however you, the trained professional, could easily have them modify different variables and you have an instant STEM, STEAM, or EDP activity.
Many of them also have short videos. Don’t overlook the opportunity to teach cultural diversity as the language, etc are slightly different from here in rural USA.
With several hundred activities you could literally have one per day – as if you had THAT kind of time!
Appropriate ages? Use your professional judgement. Most are intended for upper elementary but with the proper presentation I have used some up to grades 9, 10 and even in college! When working with the little ones though be aware of safety hazards.