Ask any grade school student what a simile is, and they will tell you it's a comparison using "like" or "as". Somehow, they all memorize that definition, and can easily identify one. Ask any grade school student what a metaphor is, and somehow, they get tongue-tied and confused. Metaphors are just more difficult for kids to understand. Both make comparisons, but the use of "like" or "as" make them more easily identifiable.
I tried to find an easy activity for my 7th grade Language Arts Lab classes of both general and special needs kids to do to tune them into the difference between the simile and metaphor. I came up with "Metaphorically Speaking". I had them compare themselves to 4 non-human things by looking at the traits of that item. Then, they wrote out the statement for the comparison and illustrated it.
The results were funny. Some of the students came up with some great comparisons. One of my favorites was a very quiet young lady who compared herself to a taco, because she was "spicy". The posters hanging in the hallway gave a chuckle to the teachers walking by, for sure. Here are some of the posters from the classes:
I'd have to say that the students definitely understood the difference between a simile and metaphor once they did this activity. I asked a few of them to reword their statements to make similes of them, and they were able to do it. "I am as fast as Sonic." "I smell as nice as a flower." The weekly Lab class was one of my favorite classes to teach because of activities that allowed the student's creativity to shine.