Anything smaller or the same size as him. Various humanoid species, rocks, plants, birds, insects, clothes, his combadge, beakers, food...Like I said in the form, simpler things are easier, and sometimes he's so good with them, a DNA scan will show the creature he's mimicking and not his own DNA. But humanoids are hard to perfectly mimic, as evidenced by his lack of eyebrows, lips, and halfway complete ears.
EDIT: don't expect him to shift into a Triceratops or something. At least...not a full-grown Triceratops. A really tiny, weird-looking one. :P