The 'create-a-creature' activity is a hallmark of middle and high school biology, and is a great way for your students to express their creativity and knowledge.
This activity packet provides some basic guidelines and ideas for your students to consider as they make their creatures. I found that it is best to give students the option of drawing or sculpting their creatures. It is easy to modify this document to fit different ability levels.
Students may also benefit from compiling their reports (either this packet or a separate, more realistic scientific report) and going through the process of scientific peer review.