I like to do a hard set of 10 on inlcine situp/crunch(it's sorta a combo of crunch transition into situp, don't go all the way up and then crunch some more, lower and repeat. That, and some hanging leg raises and weighted ab ball crunches rounds it out. Probably takes less than 3 minutes twice a week. Will add in some moderate weight dumbell sidebends pretty soon.