Planking, in my opinion, is a great way to tone abs in general. It also tones your glutes and arms. I highly recommend this 30-Day Planking Challenge for newbies; it helps you build up your endurance by starting off slow and increasing the intensity. It doesn't take much time and you can do it anywhere (I used to do it in my office space when I was working). It works, I promise!
I swear by doing squats! Not only does it work out your lower abdomen, it tightens your butt (