I found this Symmetry Lesson Plan online and, while it is not a "traditional" classroom lesson plan, a webquest is a great way for students to explore on their own and have fun learning along the way.
The teacher could use the Introduction as the "launching point" for the webquest and give the students some direction as to what the task is about and how much class time will be available for the students to work on the webquest.
The Task, Process, and Resources allow the student to explore many different aspects of symmetry in a fun, online way. While the student is free to explore on the web, the quest is structured well enough so that the students will stay within the bounds of the project as they work through it. There are many links along the way which will allow the student to explore in different directions, still within the parameters of the task. The grading rubric is easy for the students to understand and clearly sets the teacher's expectations for the task.
The Conclusion piece allows the instructor to tie things back together at the end of the lesson.
To me, this seems to be a great "Alternative Assessment" lesson or a lesson that gives the students a break from the rigors of the normal everyday math class and helps to make math real, relevant, and fun!