My Field Experience at Slippery Rock Middle School

This week at my field experience we have to teach a warm up lesson to our 7th grade class.  I am trying to think of some fun and unique warm up lesson plan ideas that are different than usual.  What I mean by usual is the typical "tag game" that everyone plays during warm ups and the stretches that we all get bored with.  I want to teach a fun game that kids love and will get them physically moving to get their heart rates up.  The kids in this class love running around, but the class is pretty big though.  I am talking 45-50 kids so it is hard to think of games to fit that large amount of students.  We have already played tag games in the past so I want to try something new.  Anyone have any ideas of unique warm up games they can think of?