Anything where Buffy and Spike are myths or legends but not slayers, vampires, or witches! Like fairies, or gods/goddesses.

The Truce by Eternal Red has something like that as part of the plot. WIP

I love this and wish it were completed.

Aphrodite's Pick by Dorian's Kitten on this site. Also WIP, but close to finished.

The Poet and his Muse by Addie Logan.

The Toothfairy by Sotia. Just go to author page and pm her asking for a copy, she is happy to share.

These are all here at SR.

Wild, Wicked Wishes by Fauvistfly
Summary: A birthday gift for Buffy turns out to be a genie, focused on granting three se*ually healing wishes.

I hate to suggest a WIP that doesn't look like it will be finished, but these next two sort of fit your criteria. I leave it up to you whether you want to read something that's not completed.

Lonely No More by Brat (WIP – last update in 2007)
Summary: Spike is a 'shooter' for his Uncle Eros, the God of Love, and he wants to learn more. So Eros sends him to Earth to help Buffy Summers, who does matchmaking for a living, get back on her feet and make her business what it once was. Soon, it seems Spike has gotten shot by one his uncle's arrows.

Tears in Heaven by Bloody_bint (WIP – last update in 2009)
Summary: William was an angel, but when he falls to Earth due to a mistake he made, he finds he's now just a man. Faced with a brand new body, new emotions, and new experiences, he wants only to make amends for his error. With only his guardian angel to guide him, he sets out to make one Buffy Summers regain her belief in goodness, in life, and in love. As he gets to know her, though, he starts to wonder if maybe the heaven he's been trying to get back to is now right in front of him.