I got this sequence years ago from a former instructor and it keeps my class groaning!
Have everyone place their hands under their butts to support their lower backs, pull chin to chest and lift both legs approximately 6 inches off the ground and hold. Then, do a set of scissors and back to 6 inches. Next, knees pull into chest and back to 6 inches. Finally, alternate leg switches and back to 6 inches.
At this point I tease them and have them lift to 18 inches, 12 inches, 6 inches, etc. You can make the sets as long or as short as your class can stand - I definitely make them hold those legs up though for a while. Great for the core but make sure to stretch the lower back after.