Historically, according to the Orphic hymn to Leto, myrrh is offered to her as it tends to be for most chthonic deities. Myrrh has a history of being used as funerary offerings and is at times also offered to her son in this same context. Unlike the stringent smell of frankincense, myrrh has a subtle sweetness to it.
As part of my doxa I also offer her raw honey, honey-wine and mead. Mum are my preferred offering of choice, and not just because it is the flower of November, the month I was born, but rather because of its rich funerary associations. As mentioned previously, as mettle us sacred to her this would also be an appropriate offering. As would imager of quails, wolves, frogs, shrew and serpents.