Personally, Flower and plants is ok. But bicycle etc should be kept it the house instead.
"Common" corridor is considered Public area so keep the area clear and clean.
i also know incence holder etc, as long as it don't block access and not too unsightly, it should be forgiven.
If you have any concern with your neighbours should just voice it out.
I went up several time to my upstair neighbour(at least 3 times) for inconsiderate behavoiur they cause.
eg. children bathing water just out of toilet window, throw cigarette butts, etc at the cloth hanging area
I went over the house and caught them red handed and confronted the owner.
Due to this i have clothing damage by cigarette butt, need to rewashed the laundry etc......