I just found this out recently and thought I would share. In one challenge, you can bring a Sims’ remains to the science lab to try to resurrect them. They seem to always come back as a playable ghost. I was able to create a relationship with my ghost and even have ghost children. But I didn’t know how to bring them back alive or to their bodily, corporeal form. I saw on a forum that the Ambrosia dish will do the trick! Luckily that family already had a master chef who had high fishing skill, and a full garden. They had plenty of life fruit, and with a bit of patience she caught the deathfish. She made the ghost Ambrosia and it worked!