Cheap enzyme assay for a high school lab
There is the classic potato starch/iodine/amylase experiment:
http://dailynexus.com/2016-08-31/chemistry-101-the-classic-potato-iodine-blueblack-experiment-finally-has-an-explanation/

From memory, when you add iodine to a small piece of potato, the potato starch reacts to produce a dark blue/black colour.  The amylase enzyme in human saliva can break down the starch and remove the colour, which is effected by spitting into the test tube.

You might need to try titrations to work out how to do number 3.