Cats

I like covered litter boxes with the door flap. If you've never had cats before, one thing to keep in mind is the number of cat boxes that you'll need. Typically people recommend N+1. For two cats, I would have at least 2 litter boxes. Besides, cats like having options. With respect to the kind of litter, avoid lavender-scented litters as lavender is a natural cat repellant. Also avoid silicone-based litters as it dries out their coats.

You could also try a top-entry litter box to keep the dog out of it. I've had success with using baby gates to bar the dog from the catbox area.