Set the theme and ask pupils to design a garden on paper beforehand. You could use SketchUp or CAD if you have them available. Examples could be linked to the curriculum such as Dig for Victory, wildlife gardening, dinosaurs or desert gardens.
Link the garden to a favourite book with younger children.
Flat containers such as shallow dishes or seed trays
Potting compost, plants and seeds
Collected natural materials such as moss, pines cones and twigs
Miniature figures or toys fitting the theme
Modelling clay, lollypop sticks and gravel