What should I not use as a cat topper?

It goes without saying, but avoid anything that’s toxic to cats. This includes onions (and members of the onion family such as shallots and chives), grapes and raisins, chocolate, alcohol and food containing xylitol. If you think your cat has eaten something toxic, please contact your vet. 

You should also avoid anything you know your individual cat is allergic or intolerant too and avoid things you know your cat dislikes.