One of my favorite Lenz's law and cons of energy demos is a simple hand generator like a Genecon. Have the generator disconnected and let a student spin it. While they are spinning it connect it to a bulb (or other resistor). You can even short the leads. The torque (and power) required to maintain the speed dramatically increases.
I believe it's an effective 1st and 2nd law of thermo. demonstration, too (can't get something for nothing and you can't break even) because the effort required to produce a current in the bulb to heat the filament is greater than not producing a current (open circuit). You could even add bulbs in parallel and demonstrate further increases of effort. Then if you stop the turning torque, the current won't keep the generator going. You might need to show that one generator will turn another generator, and reversing the turn direction (or the lead polarity) will reverse the "motor" side.
I once had a biologist ask me (not curiously but thinking he had a great idea) why we didn't have fans connected to generators in the grills of cars to use the wind of the cars motion to improve gas mileage or charge batteries in electrics or hybrids. I tried to explain Lenz's law, etc. He didn't believe me. So, I got the hand generator and a 1 F capacitor and let him turn it, open circuit at first, then connected the cap. Of course, he had to turn with more effort. I then explained that this resistance to turning would increase the aerodynamic drag and lower the gas mileage. His response: "But it would be different with a fan on a car because you're already moving." ARRRGGHH!