First thing I think of is [name_f]Sunday[/name_f], which may not be an “established name” honestly I think [name_f]Sunny[/name_f] is fine on its own! But here are a few more:
[name_f][/name_f][name_f]Summer[/name_f]
[name_f][/name_f][name_f]Savannah[/name_f]
[name_f][/name_f][name_f]Sonnet[/name_f]
[name_f][/name_f]Soniva
[name_f][/name_f][name_f]Sonora[/name_f]
[name_f][/name_f][name_f]Cassandra[/name_f]
[name_f][/name_f][name_f]Solena[/name_f]
Here’s a thread where someone asked the same question. You may find some ideas there as well! Good luck! Full Name for Sunny?