For my warmups lately, instead of jogging around the block and hoping the kids aren't on fire when I come back to the house, or the house isn't on fire, or the dog, I needed a warmup that wasn't burpees, so I've been practicing double unders with the jump rope.
Those are very tricky because unlike a lot of exercises, you can't fake/bad form your way through it. If you screw it up, it's over, full stop.
And as full disclosure, one spring, for sports training class, we did jump rope as our aerobic activity all spring, and all we used was a crummy piece of rope not some fancy jump rope. Long story, but at my High School, your P.E. Class could be sports training, so that if you were on the football or, in my case, wrestling team, you could start your sport an hour before school got out. When you weren't on a team, you had general conditioning training.
I took a lesson from one of the crossfit videos where you practice them by doing single double unders, like so: hop hop big hop (and hopefully a doubleunder). So, getting singles was pretty easy, to get, but then I moved to hop doubleunder hop, which is harder, but last night for some reason, I managed to get 5 doubleunders in a row. Good stuff that.