My cat can't sit on the toilet without falling in

Er - tried cleaning the floor and then using cat repellent (not sure if it repels humans too!) or citrus oil? (You know, like you put in burners)

Or - sharp objects, like the spiky things you put flowers onto to make arrangements all over the area? (Don't forget it is there!)

Little smeggers are creatures of habit - and there seems to be something about shower drains that sings "lavatory" to them!