Various other cage accessories can be placed in a rat's cage to provide extra stimulation and a more interesting environment for the rat.
Mineral and Salt Blocks
Many Pet Shops stock mineral blocks and salt blocks for rats. These are not really necessary for the rat's well being.
Old cardboard tubes or toilet roll tubes can be provided for the rat and it will enjoy using it as something to climb through, on or nest in as well as chewing it to pieces.
Wood gnaws are available in Pet Shops or you can provide your own by supplying the rat with a piece of apple branch. These provide the rat with something on which to gnaw in order to keep its teeth trim.