Protection From Death
Go to Pleasant Rest Graveyard, and near the gate, there is a strange bush called a "Death Flower Bush" - Harvest the flower from it.
If you keep this flower in your sim's inventory, when you die the grim reaper will trade it for a second chance at life.
This flower will not reappear, but there are Unknown Special Seeds (requires level 7 Gardening to plant) dotted about near the graveyard (and a few other places) which may grow another Death Flower (and sometimes more if your Sim has the Green Thumb trait to revive the plant.)
Kill Your Sim
To kill off your sim make a swimming pool then send him or her in, then remove the ladders and diving board.
