I do modified hanging leg lifts to the side. This target mainly your obliques, but also your rib muscles, AND your abs.
Like:
Another example:
That site is great cause it also has little clips to show you how to do it, not just pics.
I do a combo of both. I don't quite go up to my chin, but I bring it up about to my chest, and you can feel the burn in your abs, obliques, and rib muscles. If you contract your muscles to bring them up, not just swing your legs up there, I bet you'll only get to about 20 (10 per side, alternating). It's also a good warmer/exercise for your hip flexors.