I was reading into Magic Jar. It is used to take over someone's body and you have total control. If the body dies while your original one is nearby, you just get your own body back:
If the host body is slain, you return to the magic jar, if within range, and the life force of the host departs (it is dead).
This raises the question:
That very much depends on what you mean by "in game."
Certainly:
PC: "I fall on my sword."
DM: "Okay. You die."
No.
At least, not guaranteed at every table by a common understanding of the rules.
Coup de grace requires that the target be helpless before it's an option, and whether you're helpless to yourself is going to split the audience of DMs down the middle, more or less. Some will say no, you aren't helpless to yourself ("heck, people flinch away from their own razor!"), and others will say sure, go ahead and coup yourself out.
The combat actions and options made available by the game just aren't intended to cover every possible violence that a PC could commit. If you're alone, just you and a basket of puppies, and you pick it up and hurl it over the cliff, do you have to roll initiative? No — there's nobody there to contest with for the initiative. There's no action involved that's combative. At most it's a skill check, if for some reason there's the possibility of fumbling the throw and not disturbingly dispatching the puppies. It's not even a coup de grace.
When you're dealing with a character committing suicide (for whatever reason), unless it's suicide-by-battle the combat rules aren't engaged by the action. It's just violence, tragic or senseless, committed by the character against themself, vetted by the DM as possible and properly executed, and observed by your fellow players as a contribution to the shared imaginary space you're all consenting to create.
The conditions for being helpless against yourself are interesting. In one hand, you are completely at your own mercy, giving you the advantage over yourself, allowing for a Coup de Grace to occur. However, because you have the advantage over yourself, you aren't considered helpless against yourself.
It is probably easier to drink 10 doses of CON poison and reduce your CON to 0. If I remember correctly, you can intentionally take 1 on any saving throw to roll an auto-fail. If not, just drink more doses until you fail 10 times.
Alternatively, you can just throw every bead from a Necklace of Fireballs up in the air and take roughly 45d6 of damage (depending on which level of necklace you used).
Set up a pit trap and jump down it. Make sure it's deep enough for you to die at the bottom.
As you asked for other ways to kill yourself:
Under the assumption that you can choose to fail saves against spells you can cast suffocation on yourself and fail all three saves. The problem is that you don't leave the target body until it is dead, which means you can't do anything for the time your robbed body suffocates to death.
If the body has spell resistance you need to either make the CL check or to lower the spell resistance. The latter would mean it takes one additional round.
Other save or die spells should equally do the trick but I don't remember any right now.
