I believe the best way to stimulate a baby/child is simply by taking them out in the world.
A child can learn that a bus is a bus by having a flashcard waved in front of their face; or they can learn that a bus is a bus by riding on one, they also then learn how a bus sounds, that it can go fast or slow, that you have to pay to ride on the bus etc. etc.
A child can learn that a dog is a dog by having a flashcard waved in front of their face; or they can watch a dog chase a ball, they can listen to a dog bark, they can pat and touch a dog.
Admitedly children in HK aren't going to be able to learn about a giraffe or a lion (and many other things) by going out and experiencing one. But in that case, a good book or a soft toy is a much better experience than a flashcard which can't be heard, or touched, or smellt!
Flashcards are for much older children- and can be used creatively, for example, using several flashcards to piece together a story- but not for babies!