Well, almost anything flat and relatively thin. Just make sure that whatever is being used is narrower than the width of the rollers.
Brass sheets that come pre-textured for use in jewelry making can be used to make a number of impressions before they no longer make crisp designs. They will still be usable for jewelry making.
Inexpensive laser cut cupcake wrappers make great impressions on metal. Shown here is a ¾" wide 18 gauge strip that is 6" long. After rolling, it measured 6½" long. Since my rollers are narrow, the wrapper was cut down.
Mirror images can be obtained by sandwiching a design between two pieces of metal placed in a folder.
Imprint an allover pattern as in the pieces on the left. With the pieces in the center and on the right, the design was cut out, placed on the metal and run through the mill. Then the pieces were cut to the desired shape, center, or with a border, right.
Heavy weave ribbons (remove metal edge wire), plastic net sleeves from onions, turkeys, etc., crushed paper grocery bags, designs cut from cardstock, lace, feathers, etc., and anything you can think of will probably work. Pinterest is full of ideas.