Although he's labelled weird as a Scientologist Tom Cruise is meant to be really nice too on set and shit like that - for a pretty much superstar keeping your head with that level of fame is quite a thing.
Keanu takes good bloke to a whole new level though.
I don't consider Cena an "actor", not really, yet. As for Keanu, I heard that his total gross he "made" for the Matrix movies was something over $100 Million, yet donated like $85 million of that to the special effects people that worked on the movies because he felt they're the ones who really made them great.
There's a ton of other Keanu stories out there too, far more than just being a good dude. But donating 80-90% of the $100 mil plus you made on the Matrix trilogy to special effects crew might be up there with as many Make a Wishes as Cena has granted.