I remember a fic where Spike gives/buys Dawn a gift - something to do with her being the key. I think it might be directly after she finds out she is the key.

It might be a Christmas present.

Could it be coincidences series by all4spike
Not sure which story its in but spike gives dawn a key necklace as a kinda good luck gift and buffy gives her a nice shiny dagger!

Thanks. I found the part you mentioned.

It could be the fic but I thought the gift was more drawn out with the meaning of Dawn being the key. I was thinking more along lines of Spike giving Dawn the gift because of her not "just" being the key.

Thanks again though.

I think it could be Somniloquy by Gwyneth Rhys. On EF and AO3.

Summary: What if Spike had actually saved Dawn, and thus saved Buffy, in The Gift? Does being a reluctant hero change your status with the people who are reluctant to admit your heroics? And would Buffy know it if the ending of her story was supposed to be different? 

In the second part Spike offers Dawn a key with this note: «This doesn't open anything. It just is.» Dawn was worrying about her purpose in life after the battle with Glory. Spike told her that she would be appreciated for being just her and not the key.

Thought I'd drop a word if anyone's interested in this topic.