Working Out

B/c of the holiday and the busy week, I didn't get to the gym till Friday this week. Rather disappointing, but at least I went once. And I discovered in the week of no working out, I didn't gain any weight. I should clarify, I have to do certain exercises in the morning so my back isn't screaming at me all day, which includes hip thrusts, pseudo push ups (legs flat and only moving the upper half of my body), and if I have time I do some exercises to strengthen my thighs so my left thigh doesn't hate me. I also walk up and down Old Main Hill and the stairs every day to and from work, and every once in awhile I get a little cardio playing with the Beamo (a large, spandex/fiber glass frisbee) with my coworkers.
I've realized that while I want to work on my midsection, there are exercises that kill my back. Doing the hip crunches, or whatever they're called, hurts my back a lot. I tried just doing leg lifts, but that wasn't much better. Does anybody know of any complete ab workout that is gentle on the back? I want to focus mainly on my lower abs, but I'll take an all encompassing workout so long as it doesn't kill my back. Oh, and my back doesn't like the one back workout I do either. The morning after I do them my back is really upset. The sad thing is only when I can pop it first thing in the morning does my back get relaxed. My stretches and exercises help, but it's the popping that makes everything good. I can generally pop my back within an hour or two of being at work, which helps. So any ideas, feel free to pass them on.