Drowning. Preferably with a lot of alcohol/some benzos involved. Unless a note or such is found, most drownings are ruled accidental. Especially if an intoxicant is found in your system postmortem. Even professional swimmers occasional succumb to accidental drowning pushing their bodies too hard. Shallow water blackout is one such method. There is also an area after large dams notorious for drowning people due to water currents forcing you under with no chance of being rescued. People in kayaks/canoes often die when they get sucked in after getting too close out of curiosity. No-one sane will even attempt to rescue anyone caught there as it is so bad any would-be rescuer will also drown. They can't even recover the body unless it somehow makes it out of the current.
Car accident is not a good idea. There are signs like lack of serious attempts to stop the car/swerve to avoid an animal+lack of proof of said spooked animal nearby that can and will show it was intentional. Plus the very serious risk of hurting or killing someone who did not have intent to CTB. Any plan or method with risk to others is to be frowned upon heavily.
CO/ SN poisoning are likely to at least raise suspicions. Accidental poisonings do happen, but are far outweighed by intentional ones and they raise a lot of suspicion of either suicide or homicide when they happen.
Also, newer generators tend to have a built in shut off for if CO levels start to build up. Tampering with that makes it immediately obvious. A running car (with heat going and power out in a cold area) can be seen as accidental and happens often enough. Personally it seems too slow and unreliable though. Surviving CO poisoning causes a lot of damage.
To look for ideas, do research into accidental deaths and break it down and try to find something you consider reliable enough.
Intentionally CTB'ing is hard enough as it is. Making it look like an accident multiplies the difficulty greatly.