Science is a subject that tries to explain how the world works; it is a simple fascinating subject that I have helped teach in schools in several countries. Many schools worry that it can cost too much time and money. There are numerous Science Fair Projects that are free (or incredibly cheap), easy to set up, and yet even a parent can comprehend!
How about something using insects? Entomology is a very well established science and bugs are found all over the world. Have your child catch a few and examine them closely. Projects include: a collection showing scientific classification; Shape and different types of wings; catching larva and showing how they change to adults. You could also research how insects have spread disease, caused famines, or been used as a food supply to prevent starvation. Children are usually fascinated by these little crawly things and 99+% are harmless (they can set up a display to show the dangerous ones if any are around).
Feathers are also usually free and easy to collect. Set up a project showing how they can repel water and how they are specialized for different types of uses; a tail feather is very different from one removed off the wing. Did you know that birds can even be sexed from the feathers, and that is from the shape rather than the color! They can also put the feathers in ink or dyes and demonstrate the vascular systems. Feathers are fantastic for science.
Physics projects can frequently be made using old parts of televisions or Computer Monitors. Check out those flyback transformers, transistors and resistors, but be careful using those capacitors. Hook everything up to a 9 volt battery to limit the dangers, but you can still get quite a shock at what can be done. My son built a plasma arc welder and a coil gun, didn’t shoot much but still was and interesting and fun project; we only had to buy the battery!
More physics can be amply demonstrated using old paper; children love paper airplanes! Have them build planes using different shapes and weights of papers, then try flying. They can discuss air flow as a result of texture or resistant and distance flown. One shape may be incredibly fast for a short distance, but a wide winged glider goes much farther as it floats on the air.
Cheap or free projects are only limited by the imagination. Collect a bird or fish skeleton and put it together using thread, it becomes like a puzzle. Dissolve things in water and show how thing float differently. Try studying a few leaves! This is all science, and it doesn’t have to be expensive!