Inspired by the song Scotty doesn't know by Lustra, I can see this going 2 ways both very different to each other so I'll lay them out below:-

Option 1:- Spuffy vs Riley

Either season 4 or 5 [Preferably early in the B/R relationship season 4 but up to the author], Buffy and Spike get together earlier then canon but during the time she's still dating Riley, instead of dumping Riley they find it thrilling to sneak behind everyone's backs, Buffy often lying to Riley about where she is or who she's with, hiding their familiarity behind faux antagonism and verbal battles.

Or

Option 2:- Spuffy Vs Angel 

For whatever reason you want, Spike and Buffy have been sent into their past [Season 2, Pre-Innocent], in their old bodies and must come up with a way to return to their own time or alternatively must make enough changes to the future without changing to much of the past that they are unaware of what will happen, all while trying to hide their relationship and knowledge, in doing have to sneak around behind everyone's back, particularly Buffy's then boyfriend Angel.

Option 1 Rules:

Must Have:- 

• Include a scene following the all scenes described in the song [back of Car [van], while under Spike, while on the phone, in a parking lot, front lawn in the snow, on her birthday]
• Must use both excuses used in the song [at church and shopping]
• Infidelity 
• Spuffy HEA/HFN

Must Not Have:- 

• Them being caught by Riley or their Friends until all of the above scenes play out 
• Explicit Riley/Buffy Scenes - yes I still want them dating but I don't want to read them getting it on
• Character Death [Unless it's canon]
• SA of any kind
• Overly violent Spuffy relationship [anything above kink level is to much]

Can Have (Optional):-

• Kink Discovery 
• Buffy giving Spike Blood
• Spike getting the chip out
• Claim
• This conversation in some form:

Buffy: You're 'the second worst thing to ever happen to those orphans', what does that mean?
Spike: It means i was second worst thing to happen to those orphans.
Buffy: but what’s the first worst thing?
*Awkward pause*
Spike: Buffy, they...they weren’t always orphans.
Buffy:

Option 2 Rules:

Must Have:-

• Include a scene following the all scenes described in the song [back of Car [van], while under Spike, while on the phone, in a parking lot, front lawn in the snow, on her birthday]
• Must use both excuses used in the song [at church and shopping]
• Infidelity
• Spuffy HEA/HFN

Must Not Have:-

• [If permanently in the past] Angel, Dru or Friends discovering the relationship until all above scenes play out
• Them being discovered as time travellers [in the case of them only being in the past until they find their way back]
• Angel/Buffy Explicit scenes, yes Buffy is still be with him to keep the timeline secure but Innocent shouldn't play out the same/should be back to their time by then
• Explicit Spike/Dru, yes he'd still be with her for the timeline but we don't need to read them together intimately 
• SA of any kind
• Overly Violent Spuffy Relationship [Anything above kink level or to keep their cover is to much]

Can Have (Optional):-

• Kink Discovery 
• Chipped or Souled Spike
• Angel Bashing [behind his back]
• This Conversation in some form:

Buffy: Would you stab your best friend in the leg for 10 million gold?
Spike: You stab me, and then when my leg gets better, we buy a big-ass house.
Angel: You can stab me too, then we'll have 20 million.
Spike: Good thinking