Although I enjoyed the small animals that were collectible in TS3, particularly the variety, I really miss the pet birds from TS2. There were so many interactions, tons of options for different birds, and they were detailed in appearance and well designed.
ETA: as I recall, you could feed them (which looked much more detailed than the feeding interaction in TS3), talk to them, and I think even teach them to speak? I think there were actually a couple of other interactions as well. And they would sometimes get loose and fly around the room.