One of the few female names my SO and I can agree on is [name_f]Serenity[/name_f]. We both love it, me for its peaceful meaning and because of the connection to his favorite show (Firefly). My only issue is finding a cute nickname! Any suggestions would be appreciated.
[name_u]Ren[/name_u], [name_u]Rennie[/name_u]
Yeah [name_u]Ren[/name_u] and Renni are the only nicknames I can think of
[name_u]Seren[/name_u] would be a perfect nickname for [name_f]Serenity[/name_f]!
Here’s a little list!
[name_f]Sara[/name_f]
[name_u]Wren[/name_u]
Renna
Nit
Nitty
[name_f]Nita[/name_f]
[name_u]Kit[/name_u]
[name_f]Kitty[/name_f]
[name_f]Hope[/name_f] this helps! 
I think that name is a very hard name to find nicknames for
[name_u]Ren[/name_u]
[name_u]Wren[/name_u]
[name_f]Sera[/name_f]
Sese (like [name_f]Cece[/name_f])
[name_f]Sia[/name_f]
[name_f]Serenity[/name_f] is beautiful
[name_u]Ren[/name_u] or [name_f]Nia[/name_f]
Seconding (or fourthing at this point) [name_u]Ren[/name_u] and [name_u]Rennie[/name_u]/Reni
[name_f]Serenity[/name_f] nn
Serrie
Errie
[name_f]Erin[/name_f]
Ret/Rett/[name_m]Rhett[/name_m]
[name_u]Seren[/name_u] (which could also lead to the nn [name_f]Star[/name_f] from it’s meaning)
Ennie
Sese / [name_f]Cece[/name_f]
[name_f]Ettie[/name_f]
[name_f]Rettie[/name_f]
I’ve always loved [name_f]Serenity[/name_f] nn [name_u]Seren[/name_u] ([name_u]Seren[/name_u] is very popular where I live).
I love the suggestion of [name_u]Rennie[/name_u] though, although it does kind of make me think of the tablets. [name_m]How[/name_m] about just [name_u]Ren[/name_u]?
I also like [name_u]Seren[/name_u], [name_u]Ren[/name_u] and [name_f]Nita[/name_f] for [name_f]Serenity[/name_f], another option (bit of a stretch) is [name_f]Rita[/name_f].
However one that I have a soft spot for although it is completely unrelated to the name except for the reason you SO likes it is ‘firefly’ I think as a nickname for a little girl it’s really sweet and can be used by everyone or just you and your SO.