You can't get rid of the henna smell. It's always going to smell like henna, and the hair follicle will probably always smell like henna at least a little bit when it's wet. I deal with it okay because it's really only a couple days after application when the smell is noticeable on dry hair.
To neutralize some of the henna scent while it's in my hair, I mix in powdered ginger root and cinnamon. Ginger and cinnamon will feel very warm on your scalp, so if you're sensitive, you might not want to try that and just tough out the henna stank. ETA: I've also heard ground cardamom seed leaves your hair smelling spicy after you rinse, but I haven't tried it. I don't go overboard with additives but sometimes they make it tolerable to leave in longer.
On another note, I notice that I don't get any better dye absorption from leaving it in for five or six hours (or fully overnight) than I do from leaving it in for two or three hours. That could just be my hair, though, as I henna my entire head every six weeks or so but really only ever need to cover my roots.
Wrap your head well with saran wrap and make a towel turban to go over that. It'll hold in heat, which will help the dye release and absorb better into your hair follicles (which means you don't have to leave it in all day/night) and it'll hold in some of the smell too.
I use flowery scents, essential oils or perfumes in my hair for a few days after I henna. It's a strong scent either way, so if you're extremely sensitive it might be overpowering, but I'd rather smell perfume than wet hay.