Sturgis: night air, the stiff scent of paper goods (such as books, or folders for processing), tequila, and cinnamon, sugar, and other assorted indicators of baked goods.
Nick: (I don't care if he's dead.) Gunpowder, blood, freshly-cleaned skin, recently-turned earth planted with roses.
Rodolphus: that deep forest-y smell cut with a lady's powder, a tinge of sweat, that strange waxy smell of lipstick that Bellatrix wears from time to time
Mina: Velvet, red wine, masculine cologne, and musk
Regulus: Tea and cream, that cold air you get when flying high above the ground, and late-summer flowers over a field
Diedre: freshly-cut Quidditch pitch grass, Gus's soap, that thin sheen of magic that she convinces herself she can smell when he comes back from the Department of Mysteries