Double Unders - Work on your rhythm, and staying upright, instead of jumping in an awkward hollow position, or picking your feet up behind you. If you're having trouble with these, focus on getting one at a time, and put a few singles in between attempts.
As Many Rounds As Possible in 12 Minutes:
30 Double Unders
20 Kettlebell Swings (70/53)
Not For Time:
100 Banded Good Mornings - Focus on keeping your back straight, and using your glutes and hamstrings to stand back up. Try to do these in 2 sets, 3 max. Use a band that's pretty light for yourself.