I would prefer some cool automation based topic..something other than the common one's like temp. or level control

Try controlling an inverted pendulum.

How about a Oven with a toxic vapor system?

You can add some more functionality like triggering SMS or email in case of HH LL this will keep the concept simple and attractive. I've seen many people get really fascinated when they see such functionality specially in teaching industry.

Suggestions that quickly come to my mind: Solder reflow oven controller; commercial coffee maker controller; programmed-routine test apparatus for cyclically testing AC induction motors or DC motors by sending signals to a VFD or DC speed controller and monitoring performance response from the motor; PID control of a ball floating in a tube while elevated by air from a PLC-controlled blower; elevator controller; anything green such as a windmill control system or tidal power generator control system.

we can help you for complete hardware and software for micro controller based temp rature and humidity control system or level control system .

try flow control using arduino

@Christopher Calonzo
What is arduino?

You try developing a system for fracking seeing as it's the latest 'new energy' industry. You could develop a system to use various chemicals blended together, drill depths, pressures, venting etc.
Good luck on your project.

Why not try to build an expert PID control system?