I've owned cats my whole life and am currently owned by two three year old rescue kitties. This is the gift basket I'd put together for a new kitty owner at the holidays.

Adoption gift certificate (optional) - these are great because it lets you "give" someone a pet while still allowing the recipient and the rescue/shelter to figure out the best possible match.

Furminator - I couldn't live without this. My cats have been exposed to it since they were little so they love it now that they're older.

Grooming Set - The Furminator is awesome but I've also found that I need combs as well as a slicker and/or nylon bristle brush. My cats actually like the Furminator a little too much, the bristle brush lets us keep brushing without going overboard on hair removal.

Nail Trimmer - Not exciting but very important.

Feliway Diffuser or Feliway Spray - Really important to have but not something new pet owners think about until there's a problem.

Nature's Miracle - We all need it at some point.

Cat Grass - Some cats like it, some cats don't so it's good to try once or twice.

Cat Dancer - So simple and yet so perfect. My cats adore this but because it's so plain it never caught my eye in the store.

What I wouldn't include: The fun stuff and the "personal" stuff. Let the new owner pick out toys, beds, blankets, and the other really fun stuff. Your basket will focus on the not so fun to shop for necessities. Also let them pick out litter boxes, accessories, and food to fit their space, budget, and kitty's existing diet. If you still want to be more generous, include a PetSmart gift card so the new owner can go pick things out him/herself.