Wow I must be a lazy parent...
How about different foods like apple, banana, etc, different body parts like foot, nose, ear, etc, and different items of clothing like sock, shirt, pants, etc?
LOL! No, literally the only reason I'm doing this is because there are outdoor cats around us, and he loves when he sees them through the window. When I read him a book with a picture of a cat in it, we can't ever get past the page with the cat picture, because he just wants to point at it. So I figured I'd make him a laminated card with a cat on it so that he can look at it. And then I figured that since he loves to look at cars but we live on a road that hardly ever has cars drive by, I'd make him some car cards while I was at it. It just kind of grew from there.
ETA - I'm hoping it was obvious, but the LOL was to your comment about being lazy, not about your suggestions. :) I'll definitely add those to my list.