I love flash cards! They have great images that grab my daughter's attention. I enjoy picking up packages that focus on one topic such as Space. If I had decided to teach my daughter about space on my own I would have taught her about the sun, planets, moon, and maybe the solar system. With this one set of flash cards that cost $1 at Target she is now learning about things I would never have thought to teach her- constellations, nebula's, dwarf planets, the space program, and Earth's atmosphere.
I encourage you to pick up cards on a wide variety of subjects because you never know what your child will find interesting. A couple days ago I picked up the animal and space cards. I was certain that my daughter would love the animal ones and I figured that I could use the space ones to teach her the order of the planets. My daughter does like the animal cards but the space ones are her current favorite. I caught her looking at them by herself today and heard her say Plubo. I asked her what she was saying and she held up a card that looked like the moon to me and said "Plubo". I find it amusing that my daughter knows what Pluto looks like but cannot pronounce it correctly yet.