You could demonstrate lighting matches and popping balloons with your X125, you could also show that it pops black and red balloons and it doesn't touch white or green balloons and you could explain why that happens.
Build a 3W IR and demonstrate it's blinding capabilities on faculty.
No just kidding.
Build a bluray, a red, and get a green and demonstrate fluorescence, visibility vs. wavelength, burning power, or refraction all while riding a unicycle.
Yea, chances are you wouldnt be allowed such a high power laser at school. Try gathering up a BR, red and a lower power green, do some colour mixing and flourescences as RA_pierce said.
The teachers should let you bring in a high powered laser if you tell them your project before hand. Heck I made a bet with one of my teachers that my "laser pointer" could light matches and pop balloons and he let me bring in the laser. I one a nice cold can of Spite. ;D
;D I nearly got suspended for bringing 3 charged 200V capacitors ( joined in parallel ) to school they werent charged to the max 600V, only to 60-80V or something Im going to take in my laser project when I finish to show my physic teacher though, should finish it by the 10th of august Im guessing.
Yeah, I would say the easiest cool thing to do would be demonstrating the popping effects on like black and red balloons compared to white ones, and explain why.