My house was saved by a smoke alarm when someone put ashes in a cardboard box when I was away. Get one or more.
Another time I discovered a burning candle on a table when the people who lit it were out. Always put a dish under them or ban them.
Another time I found a burnt towel under the front steps where someone dropped a cigarette butt.
There are too many individual hazards to name, but keeping fire and combustible materials separate is a start. Anything requiring you to remember something is a hazard — for example a wood stove that is capable of overheating if you don’t close the air intake once it gets going.
One has to patrol for hazards and have a written list of safety rules for visitors. People who would never take your car without asking you will happily run your stove.