This method requires quite some skill and preparation, but is one of the cleanest "accident" deaths.
Go to an ocean - swim about 5-7 miles out. Study a map of your chosen ocean's topography, you want to reach the spot at which the depth exceeds around a 1000 feet. For me, 5 miles is already like 7000 feet deep.
The swimming part alone is easier said than done, that's hours of swimming- you must be a strong swimmer. The only alternative is to have a boat or paddleboard or other kind of vehicle that makes it easier. A flotation device could make this much easier.
Once you're this far out, you're already as good as dead. There's very little chance of being found, and unless you make the strong effort to swim back to shore, you will just be out there till you get exhausted and die by drowning.
Drowning is a horrible to die though, so ideally bring a shotgun (almost certain death), in a sealable bag with you on your swim. Ideally, you use a Scuba-type flotation device (BCD) so that you can deflate and inflate at will.
If you have these both, along with weights in the flotation vest (again, see scuba BCD vest), you can get out to your "far enough" point, deflate the bcd so that you're staying afloat only by effortful treading, and at this point put the shotgun in your mouth pointed upwards at the brainstem, and pull the trigger.
You'll quickly sink down to the bottom of the ocean, where the chances of you ever being found will be effectively zero. Your car will be by the beach, it will appear as if you have gone for a swim, gotten taken out by rip currents, and drowned. You will have truly disappeared from the face of Earth.
This is a long and convoluted method, requiring much preparation and effort. The upside of this is that it gives you plenty of time to reconsider, you must be truly dedicated to carry this one out.
It's hard, every other way, to make your death seem accidental. Hanging is probably the easiest and best suicide method, but it unfortunately is extremely non-accidental looking. Train tracks are questionable, so is jumping. I've known people who died from overdoses on recreational drugs, but it's not clear if this was their intention or it was truly accidental. Attempting to OD or poison one's self seems like a generally unreliable method.
This method I've described should invite you to think about what a hassle it is to make death look accidental, and maybe you can ask yourself, "What does it matter how I'm found, how things go after I'm dead, if I'm... you know... dead?" Why are we worried about what happens after our death, if the whole point of doing it is that we'll be dead and therefore not care? Is it just in case there's an afterlife where we will potentially feel shame and embarrassment for going out sloppily?
Alternatively, a good "soft" or "entry level" accidental death is to spin your "mental illness" as the blame. If you are taken to be a regular person, that will sell it better, but if you have a known history of mental illness, that will work well too. Simply go off the deep end on some drugs, and make it visible somehow, perhaps via social media or, more safely, your personal recordings or journals that may be easily discovered after you go. Marijuana psychosis, is super common. Going totally psychotic off of any drug is completely accepted as a common and real thing in mental health. If you suddenly exhibit symptoms of drug-induced psychosis and commit suicide, this can be like an "accidental death", because it make it look like the drugs made you suddenly, spontaneously do it out of delusional thinking, the blame will be placed on the drugs instead of you, roughly categorizing it as an "accidental" kind of death.
edit: if you do that last method, it's very important that you not make your "psychosis" so visible that people catch you and stick you in a ward. I want to make it clear I'm recommending making it look like you're having drug psychosis; actually having it will not be helpful for anything. But yeah, it's a pretty "easy" way to "accidentally" die. Any suicide method + crazy drug-induced delusions = "accidental and tragic, a casualty of illicit substances"