Catnip is a fabulous thing for cats to play with - both dried and fashioned into a small toy, or growing fresh in the garden. Between 10 and 30 percent of all cats are unaffected by catnip - it's an inherited reaction - but most do respond, even big cats such as tigers.
The crushed leaves can also be rubbed against a scratching post to encourage a cat to stop ruining the furniture. This is because the oil in the leaves, while invisible, is still strong enough to attract cats and is more effective than catnip sprays.
Catnip toys can be kept fresher by storing them in a plastic container between play sessions, but any catnip that isn't still growing will lose its potency over time.