Colonists can engage in small-scale agriculture through the use of container gardens. Containers can be made from almost any waterproof material, and can be any needed size or shape.
Containers are mobile, and can be placed in any unused space that receives sufficient light.
Personal gardens can improve morale.
Plants grown can augment the food supplied by greenhouses and improve aesthetics.
Due to the low atmospheric pressure of Mars the containers need to be quite robust, with a structure capable of standing up to 10 000 kg per m2 of wall surface.
Otherwise, they might be used inside a larger colony space, but in such a case allotment style gardens might be even more interesting.