What you could do is react the lipase with milk fat. As the fatty acids and glycerol is released, they will lower the pH of the reaction mixture. You can observe this change through the use of the indicator phenol red. I'm assuming that you'll notice the enzyme action decreases as substrate increases because the reaction mixtures with higher amounts of substrate only lower their pH by a smaller amount, in comparison with the reaction mixtures with less substrate. So just set up like 4 test tubes and place them in water baths at 37º and place varying amounts of the substrate then add lipase + indicator etc. and record results. I think you can also use vegetable oil, olive oil and fish oil instead of the milk fat, but I'm not really sure about that. Hope that helped a bit..
Edit: Oops I meant enzyme activity will decrease with a decrease of substrate
