My P.E. methods instructor taught me this really cool game called "enemy-defender" and you play it something like this:
1. Everyone chooses an "enemy" from the group. This person is known only to you, in other words, no one else in the class knows who your "enemy" is.
2. Likewise, choose a "defender."
3. The point of the game is to always keep your "defender" between yourself and your "enemy." This can only happen when you are lined up like so:
You -------------- Defender -------------- Enemy
So that you, your defender, and your enemy make a straight line.
4. Everyone else also has an enemy and a defender. You don't know who they are. So basically everyone must move around the space (room if inside, clearly marked boundaries when outside or in a gym) continuously, because your defender and your enemy will also move to keep their respective defenders between themselves and their enemies.
5. Everyone must walk at a leisurely pace. Modifications: you can also do this on hands and knees, or using the crab crawl too. But I would only do this when there is lots of space to move around in, and preferably on a soft surface.
The game is not really supposed to stop, unless the students get all bunched up in a corner. I played this game during a rainy day with my fifth graders last semester and they didn't really get the concept - they all kept walking in circles. Students, circles are composed of infinite CURVED lines. CURVED lines are NOT straight - at least not on the cartesian plane, which is the space we are playing in.
Another, probably easier version, is the "Equilateral Triangle." You still choose two other people, and their identities are still known only to you. But in this version, you and those two people are supposed to be the vertices of an equilateral triangle.
I say easier, because maybe triangles are simpler to understand than straight lines. And truly, in Euclidean geometry, a straight line is a complex concept.