Keys are often used in magic to open up possibilities, banish things or allow access to spiritual worlds. They are symbols of Saint Peter and can be used as an offering.
It can also be worn as a talisman. Keys are also popular as part of protection charms for the home, for example in combination with chicken gods/hagstones, red yarn or blackthorns. Small Keys could also be used as an ingredient for Spelljars or Mojo Bags. There are no limits to your creativity here!
Old rusty skeleton keys are particularly suitable for this. Iron is said to keep evil magic away, and due to their age they are energetically very powerful.