The Magic Feather is a venerable literary trope, and one which can be used in a roleplaying game as well. Simply have some magical or technological item which appears to do something amazing for the character wielding it, then have them go through a quest to end with the realisation that the device does nothing, and they were performing the amazing feats unaided all along.

Just don't let them realise this too early, lest they go on a power-mad rampage that you can't stop by temporarily removing their item. You know they will.

Transcript

R2-D2: Peace Moon, here we come!
GM: Why don't we call it a night and run the mission next time?
Leia: Suits us.
C-3PO: So Pete, if I don't want to end up in prison, what can I do to save the monkeys?
R2-D2: Start a campaign. Write letters, talk to representatives of the university.
R2-D2: If they don't listen, start a grassroots protest. Get noticed. Angry students with placards always get press coverage.
R2-D2: With a critical mass of support, you can exert social pressure.
C-3PO: Is there any way to do all this quickly?
Leia: I can mention it on my video blog. I'm getting heaps more views now. Whatever you did, Corey, it's working.
Luke: Er... you still haven't sent me the login. I haven't been able to do anything yet.
Leia: But...
Han: The power was within you all along!