Since a metal filing cabinet is a basic rectangle, sewing a slip cover for it is not difficult -- if you already know the basics of sewing. You can purchase a commercial pattern designed specifically for this purpose, or draft your own using the cabinet's measurements. Making a fabric cover allows you to fully incorporate the cabinet into any room, since you can choose any print or solid that appeals to you or works with your other furnishings. Use the covered cabinet as a small table, and simply lift or untie the front cover to access your files.
Spray paint designed to work with metal is an excellent choice for covering a filing cabinet. Choose a fun, bright shade to make a cabinet to hold kids' artwork or schoolwork, or go with a more subdued shade to coordinate with an office. Remove the drawers completely and spray the cabinet and drawer fronts outside to avoid making a mess, then reassemble the cabinet when it dries.
Adding a decorative pattern or design to a cabinet won't take much time at all if you use patterned duct tape or decoupage techniques. Duct tape is available in many different patterns and comes in traditional tape or sheet form so it can be cut to shape and pressed onto the cabinet; the tape adheres easily to metal. Choose a metal friendly decoupage medium if you want to attach specific papers and images to your filing cabinet; simply cut out the images, then adhere them to the cabinet with the decoupage liquid. Your cabinet will be ready to use as soon as the decoupage liquid dries and cures -- usually in about 24 hours.
For a temporary makeover, decorate the drawers, top or sides of the cabinet with magnets or vinyl clings. Magnets stick to any metal surface, so no prep work is needed to use them to accent the cabinet. Vinyl clings can be cut into any shape or size and adhere nicely to metal without adhesive. Magnets and vinyl are good options if you are not ready to commit to a more permanent makeover, since you can remove them in seconds.