Abs

One of my favourite ab exercises of all time is laughter :)

The only time I would suggest exercise until pain is... laugh until it hurts. Besides being great for our abs; our face, heart, lungs, and disposition get a great "workout" as well.

Laughter is an instant vacation ~ Milton Berle

Another favourite exercise, of course, is yoga. Many of the postures target the core.  Also, focussing on the breath and drawing the belly up and in, while engaging the pelvic floor on every exhale, adds an intensity to the practice and can cause every posture to become "core work".  In yoga terms... this would be Uddiyana Bandha and Mula Bandha, respectively. This also helps to bring awareness to the breath, as well as the core, and is useful in preventing injury. 

Of the yoga postures, Navasana (Boat Pose) is one I often choose for its core benefits. 

Laugh, practice yoga, and breathe... rules to live by ;)