Carbon monoxide.
get a small grill. Get 5-10 pounds of charcoal. Let it burn outside your car for awhile until the charcoal is 'glowing'. Put it in your car with you, roll up windows, listen to some music and drink some beer. Look at pictures, write a note, you will be asleep within 5 mins and dead within 10. No pain. No shortness of breath. Maybe a headache or dizziness right before you fall asleep.
If you don't have a car, any badly ventilated small room will do and use duct tape to block ventilation.
