If you want to build core strength and endurance, nothing works better than the plank exercise. Holding your body in a kind of suspended animation may seem odd, but it doesn't just work the abs - you also target the back, arms and legs.
Turning the plank on it's side and adding a hip drop takes things to the next level, putting more emphasis on the obliques. This is an advanced exercise, but there are plenty of ways to modify it. One way is to stagger the feet rather than stacking them (as shown). Another option is to do the move on your hand rather than a forearm or keep one knee down on the floor and just hold a modified side plank for 30-60 seconds. You can make the move even harder by lifting the top leg up and down.
Do it right: Lie in a side plank, balanced on your forearm, feet and hips stacked on top of one another or staggered (easier), other hand behind head. Holding the torso steady, slowly lower the hips just a few inches towards the floor. Lift back to start and repeat for 10-12 reps before switching sides.