Was thinking the same thing - maybe plant some catnip outside the fence, and just give them some once in a while?
As I recall from my daughter's garden, it gets quite tall so not really a groundcover. Her cats used to nibble the leaves (it grew as a weed in her garden) but once it got taller, they were indifferent to it unless we were weeding it out, and laid it on the ground where they could get the tender young leaves. Then lay about looking mellow for the rest of the day . . .
