I like to place several kids of seeds in my science center. You can get them sometimes on sale at Wal-Mart or Lowes. Then I mix them up in a little bowl. They then have to sort them and if you can have a picture of the seeds with what grows, they can match the seed with the the plant.
They love it and it is a great way to introduce seeds and what can grow out of seeds.
You can then read them a book called: The Surprise Garden by Zoe Hall. This book is about children that sort out seeds, then they plant the seeds and it's a surprise garden because they don't know what will grow. At the end they have a salad with what grows and they love it.