I've always imagined that he didn't do NEWT level potions, purely because he would have needed an O at OWL level. In my fic, he dropped it because he couldn't stand Snape. Tonks, however, took it at NEWT because she was a hard worker and bright enough to get the necessary OWL result.
Charlie Weasley was also Quidditch captain as well as a prefect. Percy describes him as an outdoors type. He doesn't strike me as fantastically academic, and Dumbledore didn't choose prefects on their academic ability.
We don't know enough about Charlie, though, so you can go either way.
So I think the biggest change in Charlie post war is that he probably came home more often and was a bit more careful (I always viewed him as an adventure seeking guy who didn't always think about the risks). I think that having Fred die and watching how his mother suffered would just make him a bit more careful when it came to safety and stuff. I think he stayed in England for a while with his siblings, let the wounds heal, but then he returned to Romania. Maybe though, he didn't work with the very vicious dragons anymore. He probably also returned home more often than it seemed he did before the war. Especially since in a couple of years his siblings were going to start having children, get married etc.