You have a giant laser...

[whittaker]

I would use it to carv my username in a tree trunk, simply to hear someone (who knows the backstory) say: "Dude!? Why?!!"

 

[Zorg Machine]

make the lazer really really compressed and fire it on a mirror so that if bounces on another mirror and keeps bouncing back and forth.

Basically, I have a dot of pure energy, so powerful it could shoot straight through the earth and destroy/replace the sun, as my bedroom lamp...and let's see what happens when I release it into the mirror house at a theme park.

 

[Wintermoot]

I think I,ll hold the moon ransom for 1.000.000$
or I would carve my name on the moon

 

[Malgorath]

I would put it on a pedestal. Then I would set up an elaborate amount of mirrors that would cause the blast to fly around the room bouncing off the mirrors, and then hit the laser gun.

Actually, because you didn't put a time limit on how long the one shot would last, I would invent a cap for it to stop the laser and then take it off and point it at anything I want destroyed. One infinite blast.

 

[NonyaZ]

I'd fire it at an ant hill.