A guy at my gym showed me this really cool exercise that targets all of your core, and you can really feel it in your lower back and obliques.
I take a kettlebell and hold it down in front of me. I then alternate sides in how I swing it... I swing it to my left, and bring it behind the back of my head. So as it swings to the left, I'm also bringing it back behind me, and my right arm goes over my head, and bring the kettlebell directly behind my head. As it keeps swinging, I bring it back around in front of me on the right side, with my left arm / forearm passing over my head, with the kettlebell to my right side, back to down in front of me. So the kettlebell is making a circle around your body. I can't find a video of it on youtube, so sorry if this sounds complicated. It's really not, and your arms pass in a very natural motion. I swing first to the left, and next to the right. This is one rep. I like to do sets of 12.