I know I have posted about it before, but my favorite manga adaptation is The Eminence in Shadow. The LN and the manga have quite different tones. The manga is extremely silly, more like a gag series than anything else. While the novels retain a more serious dark fantasy vibe.
The two have such different vibes that it has caused problems for the series at times. I remember when the anime started airing (back when I was still on reddit), tons of people that had only read the manga were upset with how serious the show was (episode 1 was especially so), expecting something more like chuuni-Konosuba because that is what the manga is like.
I adore all three versions of TEIS (they are the only LNs I own the physical copies of), but they can all be quite different from each other at times. The LNs are medieval dark fantasy with the occasional chuuni-humor of Cid injected into them. The manga is straight gags from cover to cover where the rule of cool trumps everything else. Finally, the anime probably strikes the balance between the two, inventing whole new visual gags that only work in animation, and bringing the setting to a more industrialized era. I think it is a success story in allowing flexibility when adapting a story to fit the medium that other creators could learn something from.