100 double unders, for time.
Don't have a jump rope? GO GET ONE!!! They are just a few dollars and will provide great exercise. If you can only do one or two double unders per minute, work on them for 5-10 minutes. If you can't do them AT ALL, do 300 single jumps.
Personally, I am terrible at double unders. Doesn't matter. It is a skill I must work on.