When I worked with public school kids, they loved "getting planted"! It let's them experience what it's like to be a plant and helps develop a fascination with plants and gardening. YourGardenShow has a blog post all about it here
Other activities we enjoyed were bug searches (finding a bug and working together to identify it) and scavenger hunts. What's so fun about the garden is that the possibilities for experiential learning are endless and, at the same time, can instill kids with a lifelong interest in the garden, and fresh fruits and veggies.