Look around the classroom or maybe outside environment and look at how many lines of symmetry (a line you can draw through a shape when everything on one side of the line is the exact same as everything on the other side of the line) exist on that object. I keep a symmetry jig saw in my bag of tricks. So easy to make; pretty cardboard into geometric shapes and cut through it's line of symmetry for the kids to put together. I just put it up on the board with blu tac and get the students to put it together in turns.
I also like to refer to a line of symmetry as a mirror line because if you were to put a mirror on the line, the picture would be the same.
Triangles have one line of symmetry/ mirror line... where is it? Have a look with the kids. Show them where it is and isn't. How many on a four pointed star? A square? A rectangle? Watch as the kids become more enthusiastic and confident finding the lines of symmetry on a shape and discuss why it is or isn't a line of symmetry amongst themselves. Get them to double check a rectangles lines of symmetry with a mirror if they believe the rectangle has a diagonal line of symmetry.
Do points on a star correspond with lines of symmetry?
What about a parallelogram? Make predictions and check with a mirror.
It should take little time to make a worksheet using auto shapes or even give the children some geo shapes to trace around in their books to investigate lines of symmetry.
Make sure you've tracked down the mirrors before you start and stand is a back and let the children do the investigating. Symmetry lessons are great to facilitate children's discovery learning.
It might also be nice for children to make butterfly prints to show examples of a line of symmetry at the end of the day if you end up with time up your sleeve.