Get in the half-sit-up position with your feet anchored. Maintaining this half sit-up position while doing any movement with the arms will challenge the abs, and this one hits the delts as well.
Try to keep your spine and neck as neutral as possible. Go light and do the exercise for one continuous minute, or go a bit heavier and aim for 10-15 reps.
Christian Thibaudeau used to make fun of CrossFit athletes, but since he began coaching them, they've taught him a few valuable things... along with earning his respect.