I hope that no one will ever ask me to take care of the coffee machine at work. If they did, it would be nigh impossible for me to resist running experiments on my colleagues. You see, I don't drink coffee, I drink water, and maybe tea. So, whatever I do to the machine, it doesn't influence my performance at all.
Others, on the other hand...
Note: Any type of coffee/tea is free, soft-drinks cost money.
Replacing all coffee with decaf, and all sugar with sweetener, essentially taking away most of the reasons people drink the stuff. I'll still be a nice guy though and let them keep the milk. Come on, I'm no monster.
Using stronger coffee and upping the default dosage of sugar, making even the lowest setting higher than the normal default.
Add subtle flavours to the coffee. Things like strawberry, cinnamon, vanilla or such. Subtle enough so they can be smelled, but not tasted. Adds to the mystery of the machine too!
Reprogram the machine to completely randomize your choice. No matter what you choose, getting something different. Sometimes subtly so (barely noticable), otherwise completely. Getting chocolate instead of cappuchino, etc.