i found some great exercises, that i think are safe, it's sean t's insanity cardio abs. and one of the exercises, and this is gonna sound easy, but it's not!
you sit up, with your knees bent, then lean back, and hold your arms in front of you, and basically, you use your stomach to hold your upper body up. it is beyond hard! and has really worked my abs good! it really tightens them, the first time, i couldnt even loosen them for a while, lol
also they do a few others, one where you lean that way, hands on the floor in front of you, and use your core to pick your legs up, bend them, straighten them out, then lower them and do that over. another where you get in the high plank postition (a push up) and pull each leg up and out, slowly, then you drop to the low plank postion, a push up only with your forearms on the floor, and do the same thing, then you hold yourself in low plank and just kind of move your hips up and down a tiny bit, then change to high plank and over and over. its not a lifting thing, or a situp and doesn't seem to work the muscles around your lower stomach, just the upper part.
i didnt' explain it well, but you could try looking them up on youtube.