I've been doing sit-up twists, side bends, and broomstick twists to work on my obliques. There's some simple drills that you can do while sitting, which involve twisting at your waist while holding a medicine ball.

One humbling exercise that I tried a couple months back was holding a weight straight above my head and trying to do sidebends that way. It's harder than it looks and you have to be very careful. The change of leverage that it poses in contrast to the typical sidebend can give you a lot more resistance even when using lighter weights in your workout. You can do this with any weighted object including a medicine ball. Just be careful, warm up your muscles with some dynamic stretches, and make sure that you can handle the weight.