If you just want marks, pickup up any general topic like 'Full wave rectifier' in physics and 'Medecines and Drugs' in chem. You can find all materials on google. Sit for 3-4 hours, it will be done. Readymade models are available easily in science projects shop for a meagre Rs. 300
But if you really want to do something exciting, think yourself,look around which things can inspire you to investigate upon them and make a project.
A full wave rectifier, cost 300, you can make yourself too