Great warm up that can last a long time.
This is a warm up that i use when I'm bored.
G, C, D, C.
awesome for beginners, even better if our trying to learn a new strum, and if you get bored with just this add in...
G, D, Em, C.
That's the way i recommend it, but the way i play it is...
G, D, Em, Cx2
G, C, D, C -(different strum)
(and repeat over and over as much as you want)