Gardening provides different forms of engagement for children, including designing, planting and maintaining gardens; harvesting, preparing and sharing food. Working cooperatively in groups, learning about science and nutrition. Developing their imagination, creating are and stories inspired by gardens.
Grow carrots and runner beans from seeds. Tomotoes and lettuces and herbs are great for small pots and window boxes. All easy to grow and can pick and eat them.
When we were kids I remember growing cress from seed on a bit of wet cotton wool in half an egg shell (which we had decorated to look like a face, so that when the cress grew it was the hair).
The BBC gardening page has a number of projects and activities that are great for children to have a go at;