Alright, here's the lowdown from my grind logs and what I've seen others pull — focusing on the high-frequency stuff you actually see once you start killing a pile of abyssal demons.
Most common drops you'll notice are coins, various runes (death and chaos show up a lot for me), and a steady trickle of herbs and seeds. They also drop dragon bones fairly often compared to other slayer monsters of a similar level, which is why many people bank pure profit from bones alone. Add in the usual miscellany — low- to mid-tier weapons/armor pieces, and occasional noted items — and that becomes your reliable yield when you're doing long trips.
On top of that, abyssal demons have a few headline drops that are rare rather than common: the 'abyssal whip' and 'abyssal dagger' are what most people are hunting for, but don't expect those at high rates. If you're doing slayer tasks, bring a blood rune stack or a good melee setup, and don't forget that the consistent coin + runes + bones + herbs is what makes longer trips worthwhile. Personally, I enjoy the quiet rhythm of collecting bones and herbs while chasing that one glorious whip.