I love this warmup for both the feet and the back!
1: With the right foot, press to the ball of the foot, sliding the right foot closer to the left
2: wrap the toes around to sur le coups de pied
3: flick the toes against the ground to press back to the ball
4: heel down
5-8: repeat left
1: demi plié
2: forced arch
3: elevé and stretch the legs
4-7: sweep the arm down and out, and cambré to the left, come up
8: lower the heels
Repeat starting with the left foot.
Repeat but reverse the snake, working elevé, forced arch, plié, and cambré side as you straighten the legs.