There was a main entrance gate at my old high school. There was four sets of double door that opened out, that allowed students into the campus. These doors are damn heavy and one night I was cruising with my friend and I told him, I got an idea. I refused to answer any of his inquiries.
We arrived at the high school, I climbed up onto the top of the gate. On each door was the little hydraulic system to stop the doors from slamming shut ( know what I'm sayin?) These hydraulic arms were just little planks of metal. I pushed them down with my foot. This created tension when the door closed on the two arms. Instead of sliding freely(and the door closing), the arms rub and create enough friction to hold the door open.
Now instead of the doors swinging close, they remain open daily, until they are closed for the night. Every day I would walk through them, and see every other student walking through my simple little hack that got the doors to stay open all day.
tl;dr: Hacked some doors at my old high school so they would stay open instead of closing.
Edit: My logic is: I used the doors... four times a day average. Took three seconds to open and walk through vs one second to just walk through. So two seconds to open. About 180 days of school times four years.
180x4x2/60= 24 minutes
While not substantial, I just saved a person 24 minutes of their life.