I'd like to highly recommend an original series author, whose work I mainly find on fanfiction.net. Her nym is SterJ, and this woman knows TOS canon and characters inside, outside and upside down. She's got several types of stories, one of which are "After the Credits" stories.
Some are hysterically funny and will bring a whole new meaning to the ep next time I see it (eg, "Spock's Brain.") Some are extremely poignant and had me openly weeping (eg, "The Omega Glory" believe it or not.)
She's also got "Daycare Series" stories about young Kirk and Spock knowing one another (one I read had Jimmy telling Spock they were moving to -- the planet escapes me -- the planet where Kodos the Executioner is governor.)
Any way, read her stuff. Send her a review. I've read a LOT of her works, and I've yet to find anything not agreeing 100% with characterization (outside any slash-y stuff, which is very rare with her) and canon.