umm thats a good question
all my four cats are indoor/outdoor cats and they love spending time under my neighbours trees, even though we have plenty in our own garden
This is not really a problem as my neighbours don't mind and the cats usually come home when they are called.
Is this a problem for your neighbours or are you just worried about where your cats are ? Can you tell us a little bit more about your concerns
Well my immediate thought is that even though you might put cat-nip down, its only a temporary reaction with cats, so they might play for a while then still go back to the neighbours. Also if you are out and about yourself, you cant always keep tags on them 100% of the time.
As you know cats are very independent creatures and will come and go pretty much as they please
the only thing that might reduce their wanderings is fencing, so I'm not sure if this is something you wish to consider for your situation, if indeed, you want to take the matter that far, as obviously this can be costly.
Keep us posted !