Blessed water by the door of the house.
Bring a small bottle to your parish and ask a priest to bless it, or fill it from the parish supply where one is offered.
Mount a small font by the front door, or by the bedroom doors.
Parents commonly bless children with it on the forehead as they leave the house.
The home is called the domestic church because it is where most of anyone's Christian life actually happens. Sacramentals in the house make that claim visible and turn the threshold into a place of prayer.
Blessed water for household use is attested very early and appears in the oldest Roman rituals for blessing homes.
The domestic font is the church door brought home, and it survived in Catholic countries as one of the last objects to leave a house when the faith went quiet in it.
Free. Blessed water should be poured onto the ground or into a plant rather than down a drain when it needs replacing.