A couple of ways that have caused my ruler personal injury with reasonable amounts of success: This is now a list of every way I've ever even heard of to pass your rule.
Some health effects are transmittable. That page covers transmission probabilities, and the traits page covers the exact in-game health severity. Marrying someone (or having an affair with them - seduction focus in way of life should make this easy to accomplish, if you have an appropriate target at hand) with 'great pox' gives you a pretty reliable shot at getting it. It's best if you're unable to produce any offspring, because it can be inherited.
I'm also convinced that some lunacies are contagious. I saw a ruler with great pox pass it to a spouse, a lover and a child, and then have 4 generations of offspring get 'lunatic' traits after adolescence.
As Philipp points out, the Depressed health trait is the easiest way to off yourself as it enables an option to commit suicide. Right click your own portrait and have at it. This reddit thread covers some pretty good ways to get the depressed trait, the most straightforward of which is to gain a lover (your spouse with family focus or someone else with seduction focus) and have them killed, via plot or imprisonment and execution.
Grand Hunt event chain, which requires the Hunting focus if you have Way of Life enabled. You also must no have any health conditions, including any illnesses, mental defects and demonic possession, and must be at peace. This can trigger events which give wounded, maimed and death. It's not 100% effective, but it's fairly easy to trigger; without Way of Life, you can just start the event from the intrigue menu once a year in September and October.
Going on a pilgrimage is the other event chain that can be reliably triggered an has a fair chance of personal injury. It's fairly common to become ill, but less common to die outright. I think if you pick a dangerous destination, such as a Jerusalem if there hasn't been a successful crusade, or a holy site in the west if Islam controls the Iberian peninsula, you're more likely to take wounds, maimings and death.
This also requires you to have no health conditions. Also, both can get cancelled if you go to war, so don't try them at the same time as the next two. Characters can also only go on a pilgrimage once, unfortunately.
You'll also have a regent while on a pilgrimage. A regent is a whole separate risk to the realm. Be sure to pick a good regent by giving someone your designated regent title. An ideal regent has high enough stewardship that your demense doesn't get too big, high enough diplomacy that you don't have too many vassals (usually less of a problem unless you're an emperor), likes you a whole lot, but is also liked by your vassals.
Forced abdication. This was covered briefly in the same reddit thread, but if you go on a reign of tyranny, there is a good chance that one or more vassals will resist your demands and invoke the Overthrow Ruler casus belli. If you lose this war, you will be forced to abdicate and your heir takes over. You'll know you're in the right kind of war if it's called a "War against the Tyranny" or "Revolt against the Rule" war.
The downside is your crown authority will be bumped down one level. This can be bad:
No matter what level you're at, you will lose some of your ability to raise vassal levies. Levy math is complicated, so be cautious about this unless you have a larger demense and personal levy than any two of your vassals.
If you're at medium, vassals will be allowed to fight each other when you drop to low, which could allow an already strong vassal to press claims on weaker vassals. Unless the weaker vassals are your direct allies, you won't be able to intercede. Remember, your heir just took over, so old alliances may be invalidated. As always, ambitious vassals are more of a hazard for this.
If you're at high, then titles will be allowed to pass out of the realm via inheritance when you drop to medium. This is rarer, but if you have a lot of foreign married vassals, it could be devastating.
You can always raise your crown authority as your heir, but not for 10 years. It's no coincidence that the first 10 years of your new reign will be the roughest.
Most of these next war-focused methods don't work or are more limited if you are a female ruler, especially in an Abrahamic faith.
Leading troops. Generally, being on campaign seems to be more stressful than reigning at court. You can also force your character into territories that have ongoing disease outbreaks, which increases your chance of catching a health condition.
Losing battles. This is safest if you can join someone else's war, as the penalty for losing the war is not as severe. You can still send yourself on suicide missions while using the bulk of your troops to win the war, but that can be tricky - the AI might get cocky and faceplant into your troopstack before you get enough raiding attempts in. You basically want to lead as small of a contingent of troops as you can muster and fight as large of a raging horde as you can find. The lower your martial sill, the more effective this will be. Also, detrimental traits like drunkard and possessed can increase your chance of a battlefield accident.
Both of these are more effective if you already have a health condition in general, as a 'wounded' event will result in death if your health is low enough (I think). Basically, the more negative health modifiers you have, the more likely you are to drop dead at any given time. I know you can lead troops if you're maimed, infirm, possessed or have a minor illness (-1 or -2 health illnesses), but incapable trait keeps you at home and in a regency, and the more major illnesses might keep you at home.
The danger here is getting captured. Health effects that increase your chance of injury also increase your chance of capture: possessed is the worst for this, as it actually increases your personal combat skill, which positively impacts your chances of avoiding injury and death. 1 If you get captured in your own war, the enemy gets 100% warscore instantly. If you were fighting a rebellion, then you just gave into their demands. If you get captured in an allies war, it could be good or bad. Good because being in the dungeon lowers your health and makes you more likely to die. Bad because you're in a regency for the duration, and your regent is more prone to rebellions and giving in to demands, foreign invaders, etc. See above for tips on regents.
Fighting duels. Also unlocked by Way of Life, a duel is a good way to die. Even if you have a high personal combat skill, the chances are very good that you will be wounded, maimed or killed. If you're trying to lose, just target anyone you can find with a high personal combat skill. Getting/having the Zealous trait gives you more valid duel targets.
Get assassinated. Find an incompetent spy master, or better yet, give it to someone who hates you. For a bonus, grant an enemy the Cupbearer title. This grants them an event to poison you outright, but also increases their plot power against you by quite a lot.
If no one is trying to assassinate you, try firing off a few assassinations yourself. An ideal target is high-intrigue, ambitious and deceitful, but otherwise has positive traits (so other AI's will tend to have a positive opinion of them and back their plots). They should also be relatively powerless, so they will favor assassination over rebellion (which would be worse for the realm): a baron if you are a duke or a count if you are king. Then set yourself up for failure. Pick an incompetent spymaster (or no spymaster - try banishing or marrying off your spymaster to empty the seat) so your intrigue plot power is reduced, then invite some other people with crappy intrigue skills to the plot. One of them will spill the beans, giving your target a -100 opinion modifier against you. Cancel the plot before you accidentally off them, and if they're ambitious or hateful enough, they'll soon be plotting against you. Naturally, don't imprison them if you discover any plots against you and don't tell anyone to end or stop backing a plot on your life. Repeat this with more high-intrigue targets and soon enough, one of them will get you.
Another easy way to get a targeted negative opinion is to fire someone from the council. It's a quick -20 that lasts for a while. Don't forget the 'Court Jester' honorary title for -15, and if they're your direct vassal, you can raise their levies indefinitely for a penalty of about -6/year.
The only potential downside is that a spymaster who hates you or is incompetent won't reveal as many plots. In larger realms, you'll almost always have someone plotting to off a dynasty member or plotting to fabricate claims on another vassal. If you're very unlucky, this could fracture the realm before your heir can stabilize it.
1: The exact mechanics for this are pretty vague. I've only seen some educated guesses on Reddit for documentation here and here. Could make for a good question here.
That's all I can think of that didn't seem just totally random. If you get a disease outbreak in your capital, you're slightly more likely to get ill while doing nothing, I think. But most diseases seem totally random - there aren't ways to keep epidemics under control or keep your own person out of harms way.
There are a lot more events in the game, many of which can result in injury and death, but I'm not sure how to trigger them (without cheating; console commands can probably end your life quickly and reliably - there's apparently a console command to kill a character outright).