I'm a big fan of fics where a well-timed spell, a friend stepping in to help, or someone asking the right question at the right time can solve a problem from canon.
Here are five short classic fics in that category:
1.
Apotheosis by
Ephemeral Night (Willow/Tara, General Audiences). Willow stumbles upon the secret for carrying out the Thy Will Be Done Spell in a more effective manner, and comes up with some clever and audacious to eliminate nearly every threat the Scooby Gang will ever face, and then takes it even further into the ultimate fix-it fic territory. Tagged as "fluff and crack" in AO3, and it's high quality fluff and crack.
2.
Blood by Mediancat. (Spike, Rated T). Spike and (Surprise!Character) execute a time travel plan to save Buffy in The Gift. The plan is equal parts fiendishly clever and heartbreaking.
3.
Putting the ‘Check’ in Checkpoint by
clockwork_hart1. (Gen, PG) How a well-timed question from Anya in Checkpoint could have remedied an injustice. I think that this may have been recced before, but it's delightful enough to rec again.
4.
Getting Bent 2: Wilder Than You Think by mpoetess. (Spike/Xander, Willow/Oz). Xander (who shagged Spike under the influence of Caveman Beer in a previous story), figures out what's going on with Oz in Wild at Heart and helps Oz make some better choices than in canon. Bonus Spander angst. I also like the story's appreciation of the Willow/Xander friendship.
5.
Some Enchanted Evening by Speaker to Customers. (The Trio, K+) In Season 6, Jonathan decides to cast the Ethan Rayne Halloween spell again, and the result could be a completely different trajectory for the season. Worth the read for the bit with Andrew's costume alone.