When contacting Tiamat for ritual work, what would be best to offer her that wouldn't be a dead animal (freshly killed) or bones? We often invoke her for many things but feel guilty not offering her anything.
From what I've gathered over some quick research, Tiamat is a goddess in the Babylon pantheon. She deals in chaos and in the ocean.
Some good offerings for her could be ocean related such as sea-salt or some sea-shells. If you're more into food offerings, maybe you could offer her some sea food.
Please keep in mind that these are my suggestions and I have no experience with Tiamat.